Когда я захожу в кабинет, учительница уже сидит за столом.

Russian often uses the present tense for:

• habitual actions: something that happens regularly
• narration in a vivid style: describing a scene as if it is happening now

So Когда я захожу в кабинет... can mean something like Whenever I go into the classroom/office... or it can simply present the scene in a general, vivid way.

If you wanted a clearly past version, you would usually say:

• Когда я зашёл / зашла в кабинет, учительница уже сидела за столом.

So the present here is not strange; it is very natural in Russian.

Both verbs relate to entering, but заходить is extremely common in everyday Russian for to go in / come in / enter.

• заходить often sounds more natural in ordinary speech
• входить can sound a bit more formal, neutral, or literal

So:

• Когда я захожу в кабинет... = a very normal everyday way to say this
• Когда я вхожу в кабинет... is also possible, but it feels a little more formal or bookish in many contexts

Also, заходить is the imperfective partner of зайти, while входить is the imperfective partner of войти. Both pairs can mean enter, but usage depends on style and context.

Because the sentence describes movement into the room.

With в:

• в + accusative = movement into
• в + prepositional = location in

So:

• захожу в кабинет = I go into the room
• я в кабинете = I am in the room

Here кабинет becomes кабинет in the accusative because it is an inanimate masculine noun, and the accusative singular looks the same as the nominative.

Кабинет does not always mean cabinet in the English sense. In Russian, it often means:

• an office
• a classroom
• a study
• a consulting room or doctor’s office, depending on context

In this sentence, since there is a учительница, кабинет most likely means a classroom or the teacher’s room/office.

So learners should not translate it mechanically as English cabinet.

Because Когда я захожу в кабинет is a subordinate clause introduced by когда.

Russian normally separates this kind of dependent clause with a comma:

• Когда я захожу в кабинет, учительница уже сидит за столом.

This is similar to English:

• When I enter the room, the teacher is already sitting at the desk.

Russian punctuation is quite strict here, so the comma is required.

Учительница specifically means a female teacher.

• учитель = male teacher, or sometimes teacher in a more general dictionary sense
• учительница = female teacher

Since the sentence refers to a woman, Russian uses the feminine noun.

For English speakers, this is important because Russian often marks the person’s gender directly in the noun itself.

Уже means already.

In this sentence:

• учительница уже сидит за столом = the teacher is already sitting at the desk

Its position before сидит is very natural. Russian word order is flexible, but уже often appears before the word or phrase it most naturally modifies.

Other placements may be possible, but they can sound less neutral or shift emphasis:

• Учительница уже сидит за столом = neutral
• Учительница сидит уже за столом = possible, but more marked depending on context

So the given order is the standard one.

За is a preposition with several meanings, and its meaning depends a lot on context and case.

Here за столом means at the desk/table, not simply behind the desk in a purely spatial sense.

With за:

• за + instrumental often indicates location: at / behind
• за + accusative often indicates movement to a position behind/to something

So:

• сидит за столом = is sitting at the desk
• садится за стол = sits down at the desk / moves to the desk

This is a very common Russian pattern:

• за столом = location
• за стол = direction/movement

Because the preposition за takes the instrumental when it expresses location.

So:

• за столом = at the desk / behind the desk

The noun стол changes like this:

• nominative: стол
• instrumental singular: столом

This is one of the important case patterns Russian learners memorize:

• где? location → often instrumental after за
• куда? movement → often accusative after за

Compare:

• Учительница сидит за столом. = Where is she? At the desk.
• Учительница садится за стол. = Where is she moving? To the desk.

Because сидит describes a state, not the action of moving into that state.

Russian distinguishes between:

• сидеть = to be sitting
• садиться / сесть = to sit down

So:

• учительница уже сидит за столом = she is already in the seated position
• учительница садится за стол = she is sitting down / taking her seat at the desk

English often uses sit in both senses, but Russian keeps them more separate.

Yes, Russian word order is flexible, but the neutral version here is very good:

• Когда я захожу в кабинет, учительница уже сидит за столом.

Possible variations:

• Когда я захожу в кабинет, уже учительница сидит за столом.
• Учительница уже сидит за столом, когда я захожу в кабинет.

These are grammatically possible, but they change emphasis or sound less neutral. Russian uses word order not only for grammar, but also for information structure and focus.

For a learner, the given sentence is the safest and most natural ordering.

Russian often drops personal pronouns when the verb ending already makes the subject clear.

So you may hear:

• Когда захожу в кабинет, учительница уже сидит за столом.

This still means When I go into the room..., because захожу already shows I.

However, keeping я is completely normal too. It can make the sentence a little clearer, fuller, or slightly more contrastive. So in this sentence, я is optional in many contexts, but not wrong at all.