Когда у меня слишком длинная чёлка, я почти не вижу свои ресницы в зеркале.

Here it means when in the sense of whenever or in situations where. The sentence describes a repeated, general situation, not one specific moment:

Когда у меня слишком длинная чёлка, я почти не вижу...
= When/Whenever my bangs are too long, I can hardly see...

Russian когда can be used both for one-time and repeated situations; the context tells you which one is meant.

Because Когда у меня слишком длинная чёлка is a subordinate clause, and Russian normally separates subordinate clauses from the main clause with a comma.

So the structure is:

Когда ... , я ...

This is standard punctuation in Russian.

Both are possible, but у меня слишком длинная чёлка is a very natural Russian way to express possession or a personal characteristic.

Literally, у меня means at me / by me, but in natural English it often corresponds to I have or my.

So:

• У меня слишком длинная чёлка = My bangs are too long / I have bangs that are too long
• Моя чёлка слишком длинная is also correct, but it sounds a bit more direct or emphatic.

Also, Russian usually omits есть in simple present-tense possession, so у меня есть is not needed here.

Because длинная is an adjective agreeing with чёлка.

Чёлка is:

• feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• masculine: длинный
• feminine: длинная
• neuter: длинное
• plural: длинные

Literally it means almost do not see, but in natural English it usually means hardly see or can barely see.

So:

• я не вижу = I don’t see
• я почти не вижу = I barely see / I can hardly see

The word почти softens the negation: you can see a little, but not well.

Russian often uses the reflexive possessive свой when the possessor is the same as the subject.

Here the subject is я, and the eyelashes belong to that same person, so свои is the normal choice:

• я вижу свои ресницы = I see my own eyelashes

Using мои is possible, but it usually sounds more emphatic or contrastive, as if you are stressing my eyelashes specifically, not someone else’s.

This is a very good question, because both forms can be encountered.

After negation, Russian sometimes uses the genitive instead of the accusative, especially in more formal or traditional style. So you may hear both:

• не вижу свои ресницы
• не вижу своих ресниц

In modern everyday Russian, the accusative is often used when the object is specific and definite, so свои ресницы is understandable and acceptable. But many speakers and teachers may prefer своих ресниц after не вижу, especially in careful or textbook-style Russian.

So for a learner:

• свои ресницы = common and understandable
• своих ресниц = also very natural, and often felt to be more standard after negation

Because вижу is imperfective, and this sentence describes a general, repeated state: what happens whenever the bangs are too long.

• вижу = I see / I am able to see
• увижу = I will see / I will catch sight of

Using увижу would sound like a single future event, which does not fit this general statement very well.

Because Russian uses в with the prepositional case to mean in the mirror, just like English.

• в зеркале = in the mirror
• на зеркале = on the mirror (on its surface)
• в зеркало would usually suggest motion or direction, as in looking into the mirror with some verbs

Here the meaning is that the eyelashes are seen in the reflection, so в зеркале is the correct form.

Yes. Russian word order is flexible, and different orders can sound natural depending on emphasis.

For example, you could also say:

• Когда у меня слишком длинная чёлка, я почти не вижу своих ресниц в зеркале.
• Когда у меня слишком длинная чёлка, в зеркале я почти не вижу своих ресниц.

The original order is understandable, but Russian often moves things around to shift focus slightly. A learner should not expect one single rigid word order the way English often prefers.