Learn Hardcore Russian: Эти сомнения исчезают, когда я вижу, что собеседник меня понимает. - These doubts disappear when I see that my conversation partner understands me.

Эти – demonstrative adjective, nominative plural (“these”).
сомнения – noun, nominative plural (“doubts”). Together эти сомнения is the subject of the main clause.
исчезают – verb, 3rd person plural, present tense, imperfective (“disappear”, “vanish”). It agrees with сомнения.
когда – subordinating conjunction, “when”.
я – pronoun, nominative singular, subject of вижу.
вижу – verb, 1st person singular, present tense, imperfective (“I see”).
что – subordinating conjunction, “that” (introduces a content clause).
собеседник – noun, nominative singular, subject of понимает (“the interlocutor / the person I’m speaking with”).
меня – pronoun, accusative singular, direct object of понимает (“me”).
понимает – verb, 3rd person singular, present tense, imperfective (“understands”).

So structurally:
[Эти сомнения] (subject) [исчезают] (verb), [когда я вижу, что собеседник меня понимает] (adverbial clause of time).

Why is it меня понимает and not мне понимает?

Because the verb понимать (“to understand”) in Russian takes a direct object in the accusative, not an indirect object in the dative.

меня – accusative: “(understands) me”.
мне – dative: typically “to me / for me”.

Compare:

Он меня понимает. – “He understands me.” (меня = direct object)
Он мне помогает. – “He helps me.” (мне = “to me”, indirect object)

So собеседник меня понимает = “the interlocutor understands me.” Using мне here would be grammatically wrong.

Can I change the word order in что собеседник меня понимает? For example, say что меня понимает собеседник or что он понимает меня?

Yes, Russian word order is flexible, and all of these can be grammatically correct, with small nuances of emphasis:

…что собеседник меня понимает.
Neutral order. Slight focus on the action “understands me”.
…что меня понимает собеседник.
Here меня comes first, so there is more emphasis on me:
“…that it is me that the interlocutor understands (as opposed to someone else)”
or “…that I (specifically) am understood.”
…что он понимает меня.
Using он instead of собеседник is fine if the context already makes it clear who он is. The word order понимает меня is also natural.
…что он меня понимает.
Also completely natural, very close to the original in tone.

In normal, neutral speech, что собеседник меня понимает or что он меня понимает are the most typical. The other orders are used mostly to adjust emphasis or rhythm.

Why is it сомнения исчезают and not something like сомнения исчезаются?

In Russian, исчезать is an intransitive verb: it does not take a direct object and describes something that disappears by itself.

сомнения исчезают – “the doubts disappear”.

A reflexive passive form like исчезаются is not used, because there is no agent doing the action “to something”; the doubts are simply vanishing on their own. Contrast:

отменять / отменяться – “to cancel / to be canceled”.
But исчезать – just “to disappear” (no passive counterpart like “to be disappeared”).

So the correct and natural form is simply сомнения исчезают.

Why is сомнения in the nominative plural and not сомнений (genitive), as in эти сомнения исчезают vs этих сомнений исчезают?

Because сомнения here is the subject of the verb исчезают, and subjects are normally in the nominative case:

Эти сомнения исчезают. – “These doubts disappear.”
(сомнения = who/what disappears? → nominative)

Этих сомнений исчезают would be wrong in standard Russian. You might see genitive plural with verbs in some other constructions (especially with numerals or with there is/there are patterns), but not in a simple “subject + verb” sentence like this.

For example:

У меня исчезают сомнения. – “My doubts disappear.”
(here сомнения is still nominative; у меня is just an additional phrase showing possession)

Why is исчезают in the present tense, not future (исчезнут)?

The present imperfective in Russian is used not only for actions happening “right now”, but also for:

general truths / regular situations
habitual or repeated actions

In this sentence, we’re talking about a typical recurring situation:

Эти сомнения исчезают, когда я вижу…
“These doubts disappear when I see…”

The idea is: whenever this happens, those doubts tend to disappear.

If you said Эти сомнения исчезнут, когда я увижу…, that would sound more like:

“These doubts will disappear when I (eventually) see…”

That is more like a one-time future event, not a general pattern. The original sentence is about a regular, characteristic reaction.

What’s the difference between когда and если here? Could I say Эти сомнения исчезают, если я вижу…?

You can say если, but it slightly changes the nuance.

когда = “when(ever)”, used for real, expected situations that regularly happen.
- Эти сомнения исчезают, когда я вижу…
  “These doubts disappear when I see…”
  → Suggests this actually happens regularly, a typical pattern.
если = “if”, often more conditional or hypothetical.
- Эти сомнения исчезают, если я вижу…
  “These doubts disappear if I see…”
  → Sounds a bit more like a condition: on the condition that I see that the interlocutor understands me, then the doubts disappear.

In many contexts, both are possible, but когда is more natural here because we’re describing a usual situation, not testing a condition.

Why is there a comma before когда and another comma before что?

Russian punctuation marks off subordinate clauses with commas.

…исчезают, когда я вижу…
- когда я вижу, что собеседник меня понимает is a subordinate clause of time (“when…”).
- It is separated from the main clause Эти сомнения исчезают with a comma.
…я вижу, что собеседник меня понимает.
- что собеседник меня понимает is a content clause (“that the interlocutor understands me”).
- It depends on the verb вижу and is also separated by a comma.

In Russian, almost all subordinate clauses introduced by conjunctions like когда, если, что, потому что, хотя etc. are set off by commas.

Why do we need что here? Could I leave it out and say …когда я вижу, собеседник меня понимает?

You must use что here. In Russian, when a verb like видеть, знать, понимать, думать, говорить is followed by a full clause (“that someone does something”), you normally introduce that clause with что (“that”):

Я вижу, что он устал. – “I see that he’s tired.”
Я знаю, что он прав. – “I know that he’s right.”

Without что, я вижу, собеседник меня понимает sounds ungrammatical and confusing; it would look like two separate clauses just jammed together.

So:

Correct: …когда я вижу, что собеседник меня понимает.
Incorrect: ✗ …когда я вижу, собеседник меня понимает.

What does собеседник mean exactly, and how is it different from words like друг, человек, or парень?

Собеседник is a fairly neutral, slightly formal word meaning:

“interlocutor”,
“conversation partner”,
“the person I’m talking to”.

It does not imply friendship, gender, or age. It just names the person with whom you are having a conversation.

Compare:

друг – “friend”.
человек – “person, human”.
парень – “guy”, “young man”.
собеседник – “(my) interlocutor / the person I’m talking with right now”.

So in this sentence, собеседник focuses on the role in the conversation: “when I see that the person I’m talking to understands me.”

Could I say Эти сомнения проходят instead of исчезают? Is there a difference?

Both are possible, but there is a nuance:

исчезают – “disappear / vanish”, stronger sense of completely going away, often a bit more neutral or “technical”.
проходят – literally “pass”, “go away (over time)”, often used about feelings, pain, mood:
- Боль проходит. – “The pain goes away.”
- Страх проходит. – “The fear passes.”

So:

Эти сомнения исчезают… – suggests the doubts just vanish, are no longer there.
Эти сомнения проходят… – suggests they “wear off” or “pass” with time or circumstances.

In this context, исчезают is very natural; проходят would also be acceptable but slightly more colloquial / emotional.

Why is the verb понимает in the present tense – is this about right now, or a general rule?

Like исчезают, the verb понимает in the present imperfective can express a general, repeated situation:

…когда я вижу, что собеседник меня понимает.
= “when I see that my interlocutor understands me (whenever this happens)”.

We are not talking about one single conversation only; we’re describing what typically happens in conversations in general: whenever I see that the other person understands me, my doubts disappear.

So both present tense verbs (исчезают, вижу, понимает) together create a general, habitual meaning.

Why is it эти сомнения and not мои сомнения? Could I say Мои сомнения исчезают…?

You could say Мои сомнения исчезают, когда…, and it would be grammatically correct.

Nuance:

эти сомнения – “these doubts”. This usually refers to doubts just mentioned in the previous context, or specific doubts already known to both speaker and listener.
мои сомнения – “my doubts”. This focuses more on the fact that the doubts belong to me, not someone else.

So:

If the context is “I often have these particular doubts about my language ability / my pronunciation”, эти сомнения is very natural.
If the speaker wants to stress personal ownership (as opposed to someone else’s doubts), мои сомнения makes that explicit.

In many practical situations, both would sound fine; the original just assumes the doubts have been contextually defined, so эти works well.