Learn Hardcore Russian: Уборщица сердится, когда мы оставляем кружки на столах, ведь ей трудно убирать. - The cleaning lady gets angry when we leave mugs on the desks, because it's hard for her to clean.

сердится = he/she is angry / gets angry (3rd person singular, present tense)
The -ся ending marks a reflexive / middle verb in Russian.
Literally, сердить would mean “to anger (someone)”, and сердиться is more like “to become angry oneself”.

In practice, you just learn сердиться as a separate verb meaning “to be angry”, and you always keep the -ся with it:

Я сержусь – I am angry.
Она сердится – She is angry.
Они сердятся – They are angry.

Why is it уборщица and not уборщик?

Both words exist:

уборщик – a cleaner (male, grammatically masculine)
уборщица – a cleaner (female, grammatically feminine)

In this sentence, уборщица tells us the cleaner is female. Russian often has gendered job titles:

учитель – male teacher
учительница – female teacher

So уборщица сердится means “the (female) cleaner is angry.”

What case is кружки in, and why?

Кружки here is accusative plural of кружка (a mug).

Nominative singular: кружка (a mug)
Nominative plural: кружки (mugs)
Accusative plural (for inanimate nouns like this) looks the same as nominative plural: кружки

The verb оставлять (to leave) takes a direct object in the accusative:

оставлять что? – кружки

So мы оставляем кружки = “we leave (our) mugs.”

Why is it на столах and not на столы?

The difference is case and meaning:

на столах – on the tables (location, prepositional case, plural)
на столы – onto the tables (direction/motion to, accusative case, plural)

In the sentence, the cups are lying on the tables (location), not moving onto them at this moment. So Russian uses:

на + prepositional to show where something is:
- на столе – on the table
- на столах – on the tables

Hence: оставляем кружки на столах – “we leave mugs on the tables.”

Why is оставляем (present tense, imperfective) used and not a future form like оставим?

Оставляем (imperfective, present) here expresses a repeated / habitual action:

когда мы оставляем кружки на столах
“when(ever) we leave mugs on the tables”

In Russian, the present tense of the imperfective is used for:

regular, repeated actions (habits)
general truths

If you said когда мы оставим кружки, it would sound like one specific future time:
“When we (will) leave mugs on the tables (on that occasion)...” – different meaning.

So оставляем matches the idea of “whenever we leave mugs…”.

What exactly does ведь mean here, and how is it used?

Ведь is a modal particle that gives the sentence a reasoning / explanatory / “you know” flavor.

In this sentence:

..., ведь ей трудно убирать.
can be translated as:
- “..., because it’s hard for her to clean.”
- “..., after all, it’s hard for her to clean.”
- “..., you know, it’s hard for her to clean.”

Key points about ведь:

It often introduces the reason for something just mentioned.
It can add a slightly emotional or persuasive tone, as if reminding the listener of something obvious.
It is common in spoken Russian.

You could say the sentence without ведь and it would still be grammatically correct, just a bit less “natural” and less expressive:

..., ей трудно убирать.

Why is it ей трудно, not она трудная or её трудно?

Ей трудно is an impersonal construction meaning “it is hard for her.”

ей – dative case of она (she), meaning “to her / for her”
трудно – an adverb meaning “hard, difficult”

Literally: “To her it is difficult.”

Compare:

Мне трудно – It is hard for me.
Тебе легко – It is easy for you.
Им скучно – They are bored (literally “to them it is boring”).

Why not the others?

она трудная – “she is difficult (as a person/character).” Different meaning.
её трудно – её is genitive/accusative of она, not used here. You need dative (кому?) after this pattern: кому трудно / легко / скучно / весело etc.

So ведь ей трудно убирать ≈ “since it’s hard for her to clean.”

Why is it убирать and not убрать?

Убирать and убрать are aspectual pairs:

убирать – imperfective: to clean, to be cleaning (process, repeated/habitual action)
убрать – perfective: to clean up (once, to finish cleaning)

Here we are talking about the general process of cleaning as part of her job, not one completed event. So Russian uses the imperfective infinitive:

ей трудно убирать – “it is hard for her to clean (in general).”

If you said ей трудно убрать, it would mean:

“it is hard for her to clean up (this time / this specific mess).” That’s more about finishing a specific cleaning task, not the job in general.

Is the word order fixed, or could we say Когда мы оставляем кружки на столах, уборщица сердится?

You can absolutely say:

Когда мы оставляем кружки на столах, уборщица сердится, ведь ей трудно убирать.

Russian allows quite flexible word order. Both versions are natural:

Уборщица сердится, когда мы оставляем кружки на столах...
→ Emphasis a bit more on the cleaner’s reaction.
Когда мы оставляем кружки на столах, уборщица сердится...
→ Emphasis a bit more on the condition / situation (when we do this, she gets angry).

Meaning is essentially the same; the difference is nuance and flow.

Why are there commas before когда and before ведь?

Two main reasons:

Comma before когда
Когда мы оставляем кружки на столах is a subordinate clause (a dependent “when” clause).
- Russian normally separates the main clause and the subordinate clause with a comma:
  - Уборщица сердится, когда мы оставляем кружки на столах.
  - Когда мы оставляем кружки на столах, уборщица сердится.
Comma before ведь
Ведь ей трудно убирать is an explanatory / parenthetical part of the sentence introduced by the particle ведь.
- Such parts are usually separated by a comma:
  - ..., ведь ей трудно убирать.

So the commas mark:

main clause vs. “when” clause
main statement vs. explanatory/commenting part with ведь

What is the difference between кружка and чашка?

Both can be translated as “cup”, but there is a nuance:

кружка – usually a mug:
- often bigger
- usually with a handle
- used for tea, coffee, etc.
чашка – usually a teacup / cup:
- smaller
- often part of a cup-and-saucer set

In many contexts, English just says cup, but Russian distinguishes them more often. In this sentence, кружки suggests mugs left on the tables (like in an office or classroom).