Как только начинается лекция, я выключаю телефон.

Как только is a fixed expression meaning as soon as.

Even though как by itself often means how, and только often means only/just, together they form one conjunction: как только = as soon as.

So in this sentence:

Как только начинается лекция, я выключаю телефон.
= As soon as the lecture starts, I turn off my phone.

You should learn как только as a set phrase.

Because Как только начинается лекция is a subordinate clause, and Russian normally separates subordinate clauses from the main clause with a comma.

Structure:

• Как только начинается лекция = subordinate clause
• я выключаю телефон = main clause

So the comma is required:

Как только начинается лекция, я выключаю телефон.

This is very similar to English punctuation in sentences like As soon as the lecture starts, I turn off my phone, although Russian uses commas even more consistently in these clause combinations.

Because лекция is the subject of начинается.

In начинается лекция, the thing that is starting is the lecture, so лекция is the grammatical subject and stays in the nominative singular.

• лекция = nominative
• начинается = begins / starts

Literally: the lecture begins

Начинается is from начинаться, which means to begin / to start in the sense of something begins.

So:

• лекция начинается = the lecture begins / starts
• преподаватель начинает лекцию = the teacher begins the lecture

This is an important difference:

• начинаться = intransitive / reflexive-style verb: to begin
• начинать что-то = transitive verb: to begin something

In your sentence, the lecture itself is starting, so начинается is the correct form.

Russian often uses the present tense for repeated, habitual actions, just like English can.

So this sentence usually means something like:

Whenever the lecture starts, I turn off my phone.
or
As soon as the lecture starts, I turn off my phone.

It describes a regular habit or rule.

Also, because начинаться is imperfective, its present tense is a true present tense:

• начинается = starts / is starting / begins

If you wanted to talk about one specific future occasion, Russian would normally use the perfective future:

• Как только начнётся лекция, я выключу телефон.
  = As soon as the lecture starts, I’ll turn off my phone.

Because выключаю is imperfective present, and here it describes a habitual action.

The sentence means that this is what the speaker generally does every time the lecture starts.

• я выключаю телефон = I turn off my phone / I switch off my phone as a habit
• я выключу телефон = I will turn off my phone on one specific future occasion

So compare:

• Как только начинается лекция, я выключаю телефон.
  Habit / routine

• Как только начнётся лекция, я выключу телефон.
  One future event

This is a very common Russian aspect contrast.

It is in the accusative case, because it is the direct object of выключаю.

However, for many inanimate masculine singular nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: телефон
• accusative: телефон

That is why the form does not visibly change.

Compare with a feminine noun, where you would see a difference:

• я читаю книгу
• nominative: книга
• accusative: книгу

With телефон, the case is still accusative, even though the form looks unchanged.

Russian often leaves out possessives like мой / моя / моё when ownership is obvious from context.

So:

• я выключаю телефон very naturally means I turn off my phone
• я выключаю мой телефон is grammatically possible, but usually sounds unnecessary unless you want emphasis or contrast

For example, you might use мой if you are contrasting it with someone else’s phone:

• Я выключаю мой телефон, а не твой.

Also, Russian has no articles, so телефон can mean the phone, a phone, or my phone, depending on context.

Yes. Russian word order is flexible.

You can also say:

Я выключаю телефон, как только начинается лекция.

This means the same thing.

The difference is mostly about emphasis and flow:

• Как только начинается лекция, я выключаю телефон.
  Starts with the time condition: As soon as the lecture starts...

• Я выключаю телефон, как только начинается лекция.
  Starts with the main action: I turn off my phone...

Both are natural.

Both are possible.

• начинается лекция
• лекция начинается

In Russian, word order often depends on emphasis, rhythm, and what information feels more given or more new.

In this sentence, Как только начинается лекция sounds very natural and smooth. The verb-first order after как только is common in this kind of clause.

But Как только лекция начинается, я выключаю телефон is also understandable. It may sound a little less neutral to some speakers, depending on context.

So the version in your sentence is not a special grammar rule so much as a natural word-order choice.

Both can introduce a time clause, but как только is more specific.

• когда = when
• как только = as soon as

So:

• Когда начинается лекция, я выключаю телефон.
  = When the lecture starts, I turn off my phone.

• Как только начинается лекция, я выключаю телефон.
  = As soon as the lecture starts, I turn off my phone.

Как только emphasizes immediacy: the second action happens right after the first one begins.

As written, it most naturally describes a general habit or repeated situation.

Как только начинается лекция, я выключаю телефон.
= Whenever / as soon as the lecture starts, I turn off my phone.

Why?

Because both verbs are in imperfective present:

• начинается
• выключаю

That combination strongly suggests a routine.

If you want one specific future event, Russian usually changes both verbs:

Как только начнётся лекция, я выключу телефон.
= As soon as the lecture starts, I’ll turn off my phone.

This is one of the most useful contrasts to notice in Russian.

Yes, выключаю телефон is natural and correct.

It means I turn off my phone or I switch off my phone.

Depending on the real-life situation, Russians might also say:

• ставлю телефон на беззвучный режим = I put my phone on silent
• отключаю звук на телефоне = I turn the sound off on my phone
• убираю телефон = I put my phone away

But if the intended meaning is specifically turn off the phone, then выключаю телефон is perfectly good.

The main stresses are:

• как только́
• начина́ется
• ле́кция
• я
• выключа́ю
• телефо́н

So one natural pronunciation is:

Как только́ начина́ется ле́кция, я выключа́ю телефо́н.

A few notes:

• начинается has stress on -на-
• выключаю has stress on -ча-
• телефон has stress on the last syllable: телефо́н

Stress is important in Russian, because it is not always predictable.