Когда я долго сижу за компьютером, я начинаю чаще моргать.

Когда means when. It introduces a time clause:

• Когда я долго сижу за компьютером = When I sit at the computer for a long time

In Russian, a clause with когда often works just like English when. If this time clause comes first, it is normally followed by a comma.

The comma separates the two parts of the sentence:

1. Когда я долго сижу за компьютером
2. я начинаю чаще моргать

This is standard in Russian when a subordinate clause introduced by когда comes before the main clause.

So the structure is:

• When X happens, Y happens.

Russian marks that break with a comma.

Сижу is the 1st person singular form of сидеть:

• сидеть = to sit
• я сижу = I sit / I am sitting

Here you need a conjugated verb because the subject is я.

Why not сажусь?

• садиться / сесть means to sit down, focusing on the action of taking a seat.
• сидеть means to be sitting, focusing on the state.

So:

• Я сажусь за компьютер = I sit down at the computer
• Я сижу за компьютером = I am sitting at the computer

In this sentence, the speaker means the ongoing activity of being at the computer, so сижу is the right choice.

Долго means for a long time.

In this sentence:

• Когда я долго сижу за компьютером = When I sit at the computer for a long time

It modifies сижу and tells you how long the sitting lasts.

Russian adverbs like долго are fairly flexible in position, but this placement is very natural:

• я долго сижу
• less neutral but still possible in some contexts: я сижу долго

Here я долго сижу за компьютером sounds most natural.

За компьютером is the normal Russian way to say at the computer.

Literally, за + instrumental can mean something like at / sitting behind / working at a desk, table, or computer.

So:

• сидеть за компьютером = to sit at the computer
• работать за компьютером = to work at the computer

By contrast, на компьютере usually means on the computer, emphasizing the machine as a device or platform:

• играть на компьютере = to play on the computer
• установить программу на компьютер = to install a program onto the computer

So in this sentence, since the idea is physically sitting and working/looking at a computer, за компьютером is the idiomatic choice.

Because the preposition за here requires the instrumental case when it means at / behind in a stationary situation.

Base noun:

• компьютер = computer

Instrumental singular:

• компьютером

So:

• за компьютером = at the computer

This is something you often have to learn as a fixed pattern:

• за столом = at the table
• за рулём = at the wheel
• за компьютером = at the computer

Yes, Russian often repeats the subject pronoun even when it is understood.

So:

• Когда я долго сижу за компьютером, я начинаю чаще моргать.
• Когда я долго сижу за компьютером, начинаю чаще моргать.

Both are possible.

The version with the second я sounds a bit clearer and more balanced, especially for a full written sentence. Russian does not have to repeat the subject every time, but it often does, especially for emphasis, clarity, or rhythm.

Начинаю means I begin / I start. It comes from начинать.

Pattern:

• начинать + infinitive = to begin to do something

So:

• я начинаю моргать = I begin to blink

Here:

• начинаю is the conjugated verb
• моргать stays in the infinitive

This is very similar to English:

• I begin to blink
• I start blinking

Чаще means more often. It is the comparative form of часто:

• часто = often
• чаще = more often

So:

• я начинаю чаще моргать = I begin to blink more often

It modifies моргать and describes the increased frequency of blinking.

This does not mean faster. It means the blinking happens more frequently.

Моргать means to blink.

In this sentence, it refers to blinking your eyes.

Some useful related forms:

• моргать = to blink
• моргнуть = to blink once

This is a common aspect pair:

• моргать = imperfective, repeated or ongoing blinking
• моргнуть = perfective, one completed blink

Since the sentence is about blinking repeatedly or more often over time, моргать is the right choice.

Because the sentence describes a repeated or ongoing action, not a single blink.

• моргать = to blink repeatedly / to be blinking
• моргнуть = to blink once

The phrase начинаю чаще моргать means:

• I start blinking more often
• not I start to blink once

So the imperfective infinitive fits the meaning of repeated action.

Yes, Russian word order is flexible, but different orders sound more or less natural depending on emphasis.

The neutral version is:

• Когда я долго сижу за компьютером, я начинаю чаще моргать.

Possible variations:

• Когда я долго сижу за компьютером, начинаю чаще моргать.
• Я начинаю чаще моргать, когда долго сижу за компьютером.

These still mean basically the same thing, but the focus shifts slightly.

• Starting with Когда... sets up the condition/time first.
• Starting with Я начинаю... puts the result first.

The original sentence is very natural and neutral.

It most naturally describes a general or habitual situation:

• When I sit at the computer for a long time, I start blinking more often.

Russian present tense often covers this kind of general truth or repeated experience.

If you wanted to stress a one-time event in the past, Russian would normally use past forms instead, for example:

• Когда я долго сидел за компьютером, я начал чаще моргать.

But the original present-tense sentence sounds like a regular pattern or typical experience.