Learn Hardcore Russian: Мой день быстро проходит, когда я работаю.

день проходит – the day passes, goes by, often with a sense of its duration being used up, “the day is passing (and eventually ends).”
день идёт – literally “the day is going,” more neutral, like “time is moving forward.”

In this sentence, Мой день быстро проходит emphasizes that your day is passing quickly, that it is over before you know it.
You could say Мой день быстро идёт, когда я работаю, and it would be understandable, but проходит is more idiomatic here when talking about how fast the day is passing.

What is the grammatical form of проходит, and what does it tell me?

Проходит is:

verb: проходить
aspect: imperfective
tense: present
person/number: 3rd person singular (он/она/оно проходит)

So день проходит = the day passes / is passing (in general, regularly, or now).
Present tense + imperfective aspect here describes a regular, repeated situation: every time you work, your day tends to pass quickly.

Could I say прошёл instead of проходит, like Мой день быстро прошёл, когда я работал?

You can, but that becomes a different sentence in meaning and time reference.

Мой день быстро проходит, когда я работаю.
General, repeated situation: My day passes quickly when I work. (whenever I work)
Мой день быстро прошёл, когда я работал.
One specific past situation: My day passed quickly when I was working.

So прошёл is perfective past (one completed event), whereas проходит is imperfective present (general/typical pattern).

Why is there a comma before когда in Мой день быстро проходит, когда я работаю?

In Russian, когда introduces a subordinate clause (a dependent clause) expressing time: когда я работаю = when I work.

The rule: a main clause and a subordinate clause are separated by a comma:

Main clause: Мой день быстро проходит
Subordinate clause: когда я работаю

Hence the comma: Мой день быстро проходит, когда я работаю.
You would also use a comma if you switch the order:

Когда я работаю, мой день быстро проходит.

Can I change the word order to Когда я работаю, мой день быстро проходит? Does it sound natural?

Yes, that word order is very natural and actually quite common:

Когда я работаю, мой день быстро проходит.

Both versions are correct and normal:

Мой день быстро проходит, когда я работаю.
Когда я работаю, мой день быстро проходит.

The meaning is the same. Moving когда я работаю to the front may add a slight emphasis on the condition “when I work”, but it’s a subtle difference, not a change in basic meaning.

Why do we say Мой день and not something like У меня день быстро проходит?

Both structures are possible but they are used differently.

Мой день быстро проходит.
Literally: My day passes quickly.
Normal, straightforward subject + verb sentence. The day “belongs” to you in the usual sense.
У меня день быстро проходит.
Literally: With me, the day passes quickly.
This sounds more unusual and not natural in this context. The у меня construction is usually used for things like У меня есть день (I have a day), У меня болит голова (my head hurts), etc.

For talking about your day as the subject of the sentence, Мой день is the natural choice.

What case is день in, and why?

День is in the nominative singular:

мой день – nominative, masculine singular

It’s in the nominative because день is the subject of the sentence.
The subject in Russian (the “doer” or “thing being described”) is normally in the nominative case:

День проходит. – The day passes.
Мой день проходит. – My day passes.

Can I omit я in когда я работаю and say just когда работаю?

Yes, you can say:

Мой день быстро проходит, когда работаю.

Omitting я is grammatically correct and quite common in informal speech or writing when the subject is clear from context.

However:

когда я работаю is more neutral and standard, especially in writing or in careful speech.
когда работаю sounds slightly more conversational.

Both mean the same thing in this sentence.

Why is работаю in the present tense, not future or something else?

Работаю is present tense, 1st person singular of работать.

Here, present tense is used to describe a general, repeated situation:

когда я работаю = when I (usually) work / whenever I work

In Russian, present tense can refer to:

what’s happening now (I’m working now)
regular actions or general truths (I work, I study, water boils at 100°C)

So in когда я работаю, the present tense expresses the typical condition under which your day passes quickly.

Is word order between быстро and проходит fixed? Could I say Мой день проходит быстро, когда я работаю?

You can say either:

Мой день быстро проходит, когда я работаю.
Мой день проходит быстро, когда я работаю.

Both are correct and natural.

Nuance is minimal:

быстро проходит slightly highlights the manner of passing as a property of the verb: “it quickly passes.”
проходит быстро puts the adverb after the verb; it often sounds a bit more neutral.

In everyday speech, both orders are common. No strong difference in meaning here.

How is проходит pronounced and where is the stress?

Проходит is pronounced approximately as: pra-HO-deet

Syllables: про-хо-дит
Stress: on the second syllable: проХОдит

Phonetically (in a rough English-friendly form): pra-KHO-deet, with the х like the German ch in Bach, not like English h.

Could I use a different verb like летит or тянется instead of проходит? What would change?

Yes, changing the verb gives a different nuance:

Мой день быстро проходит, когда я работаю.
My day passes quickly when I work. (neutral, standard)
Мой день быстро летит, когда я работаю.
Literally: my day flies when I work.
More vivid, emphasizes that time flies by very fast.
Мой день тянется, когда я работаю.
My day drags / goes slowly when I work.
Opposite meaning: the day feels long and slow.

All are grammatically similar, but the choice of verb adds emotional coloring and changes how you feel about the passing of time.