Когда я смотрю видео, телефон разряжается быстрее.

In this sentence, когда introduces a time clause: Когда я смотрю видео... = When I watch videos...

Because the whole sentence describes a general, repeated situation, когда is often understood more like whenever in English:

• Когда я смотрю видео, телефон разряжается быстрее.
• Whenever I watch videos, my phone drains faster.

So grammatically it is when, but in context it can feel like whenever.

Смотрю is the 1st person singular present tense form of смотреть (to watch / to look at).

So:

• я смотрю = I watch / I am watching

The verb is in the imperfective aspect because the sentence describes a regular or ongoing action, not a completed one.

Forms of смотреть in the present tense:

• я смотрю = I watch
• ты смотришь = you watch
• он/она смотрит = he/she watches
• мы смотрим = we watch
• вы смотрите = you watch
• они смотрят = they watch

Видео is one of those Russian nouns that is usually indeclinable, meaning its form stays the same in different cases.

Here it is the direct object of смотрю, so you might expect the accusative case. But видео looks the same in both nominative and accusative:

• это видео = this video
• я смотрю видео = I watch a video / videos

So the case is still doing its job, but the word itself does not visibly change.

Russian does not have articles like a, an, or the.

So:

• я смотрю видео can mean I watch a video, I watch the video, or I watch videos

The exact meaning comes from context. In this sentence, English will usually translate it naturally as I watch videos or I watch video depending on the situation.

Russian normally uses a comma to separate a subordinate clause from the main clause.

Here:

• Когда я смотрю видео, = subordinate clause
• телефон разряжается быстрее. = main clause

So the comma is required.

This is true even when English might not always make the pause feel very strong. Russian punctuation is more rule-based here.

Because телефон is the subject of the main clause:

• телефон разряжается быстрее = the phone drains faster

The thing performing or undergoing the action in the sentence is the phone, so it stays in the nominative.

Разряжается comes from разряжаться, which means to discharge, to run down, or to lose battery charge.

So:

• телефон разряжается = the phone is running down / losing charge

In natural English, you would often say:

• my phone drains faster
• the phone battery runs down faster

Russian often uses the phone itself as the subject, even though English often talks about the battery.

The -ся ending marks a reflexive verb form. But in Russian, reflexive verbs are not always literally reflexive in the English sense.

Here, разряжать can mean to discharge something, while разряжаться means to discharge / run down on its own.

Compare:

• Я разряжаю аккумулятор. = I discharge the battery.
• Аккумулятор разряжается. = The battery is discharging / running down.

So -ся here helps make the verb intransitive: the phone is not discharging something else; it itself is losing charge.

Быстрее is the comparative form of быстро (quickly) or быстрый (fast).

It means:

• faster
• more quickly

In this sentence, it functions adverbially: it describes how the phone discharges.

• разряжается быстрее = drains faster

Russian often uses the same comparative form in places where English might use either an adjective or an adverb.

Usually it sounds like a general habit or repeated situation:

• When(ever) I watch videos, the phone drains faster.

That habitual meaning comes from:

• когда in this kind of context
• the imperfective verbs смотрю and разряжается
• the general wording with no specific time markers

If you wanted to emphasize a specific moment happening right now, you would usually need more context.

Yes. Russian word order is fairly flexible.

You can also say:

• Телефон разряжается быстрее, когда я смотрю видео.

This means the same thing.

The difference is mainly in emphasis:

• Когда я смотрю видео, телефон разряжается быстрее.
  Starts with the condition/time situation.
• Телефон разряжается быстрее, когда я смотрю видео.
  Starts with the main point: the phone drains faster.

Both are natural.

Because смотреть means to watch / to look at, while видеть means to see.

Compare:

• смотреть видео = to watch a video
• видеть видео = to see a video

If you actively choose to watch something, Russian normally uses смотреть.

Yes. A few natural alternatives are possible, depending on style.

For example:

• Когда я смотрю видео, батарея садится быстрее.
  = When I watch videos, the battery dies/goes down faster.

• Когда я смотрю видео, телефон быстрее разряжается.
  Same meaning, just a slightly different word order.

• При просмотре видео телефон разряжается быстрее.
  More formal: During video watching, the phone drains faster.

Your original sentence is completely natural and clear.

Yes, Russian often omits subject pronouns when the verb ending already makes the subject clear.

So you could say:

• Когда смотрю видео, телефон разряжается быстрее.

This is natural in conversation, because смотрю already tells us the subject is I.

However, including я is also perfectly normal. It can make the sentence a bit clearer or slightly more emphatic.