Увечері я сідаю на диван і дивлюся серіал.

Увечері means in the evening or in the evenings, depending on context.

It is an adverbial form built from вечір (evening). In this sentence, it sets the time for the whole action.

Увечері and ввечері are both used and mean the same thing. The choice often depends on pronunciation flow and style. Ukrainian commonly alternates у / в to make speech sound smoother.

So:

• Увечері я читаю. = In the evening I read.
• Ввечері я читаю. = same meaning

In Ukrainian, the present tense is often used for habitual or repeated actions, just like in English:

• Увечері я сідаю на диван і дивлюся серіал.
• In the evening, I sit down on the couch and watch a TV series.

This does not have to mean that the speaker is doing it right now. It can simply describe what they typically do in the evening.

So the present tense here is best understood as a routine/habitual present.

This is a very common and important question.

• сідаю = I sit down / I am taking a seat
• сиджу = I am sitting / I sit

So:

• сідаю focuses on the movement into the seated position
• сиджу focuses on the state of already being seated

Compare:

• Я сідаю на диван. = I sit down on the couch.
• Я сиджу на дивані. = I am sitting on the couch.

In your sentence, сідаю is used because the action is I take a seat on the couch, and then I watch a series.

Because Ukrainian uses different cases depending on whether you mean:

• movement toward a place
• location in a place

Here, сідаю на диван involves movement: you are moving onto the couch. So диван is in the accusative case.

Compare:

• Я сідаю на диван. = I sit down onto the couch.
• Я сиджу на дивані. = I am sitting on the couch.

So:

• на диван = direction/destination
• на дивані = location

This is a very common pattern in Ukrainian.

In сідаю на диван, диван is in the accusative singular.

For an inanimate masculine noun like диван, the accusative singular usually looks the same as the nominative singular:

• nominative: диван
• accusative: диван

So even though the form does not change, the case is still accusative because the grammar requires it after a motion idea like сідаю на...

Because the verb is дивитися, which is the normal dictionary form meaning to watch or to look.

The -ся ending is a reflexive marker, but in many Ukrainian verbs it is simply part of the standard verb and does not always translate as a literal English reflexive like myself.

Here:

• дивитися = to watch / to look
• я дивлюся = I watch / I am watching / I look

So дивлюся is just the correct 1st person singular present form of дивитися.

Because in Ukrainian, дивитися can work in two different ways:

1. дивитися + direct object = to watch something
2. дивитися на + object = to look at something

So:

• Я дивлюся серіал. = I watch a TV series.
• Я дивлюся на диван. = I look at the couch.

In your sentence, the meaning is watch a series, so the direct object серіал is used with no preposition.

Серіал is in the accusative singular, because it is the direct object of дивлюся in the sense watch.

Like диван, серіал is an inanimate masculine noun, so its accusative singular looks the same as the nominative:

• nominative: серіал
• accusative: серіал

So the form does not change, but grammatically it is accusative.

Yes. Ukrainian often omits subject pronouns when they are already clear from the verb ending.

So both of these are natural:

• Увечері я сідаю на диван і дивлюся серіал.
• Увечері сідаю на диван і дивлюся серіал.

The verb forms сідаю and дивлюся already tell you the subject is I.

Including я can make the sentence:

• a little clearer
• a little more explicit
• slightly emphatic, depending on context

No, Ukrainian word order is fairly flexible. The sentence as written is a neutral, natural order:

• Увечері я сідаю на диван і дивлюся серіал.

But other orders are also possible, with different emphasis:

• Я увечері сідаю на диван і дивлюся серіал.
  Slightly more emphasis on I
• На диван я сідаю увечері...
  More marked; emphasizes onto the couch
• Серіал я дивлюся увечері.
  Emphasizes the series

So the original version is a good standard pattern, but it is not the only possible one.

Yes, серіал normally means a TV series / television series.

So:

• дивлюся серіал = watch a series / watch a TV show

If you wanted to say a film/movie, you would usually use фільм:

• Я дивлюся фільм. = I am watching a film.

Then you would usually use сиджу, not сідаю:

• Увечері я сиджу на дивані й дивлюся серіал.

That means something more like:

• In the evening I sit on the couch and watch a series
• or In the evening I am sitting on the couch and watching a series

So the contrast is:

• сідаю на диван = I sit down onto the couch
• сиджу на дивані = I am sitting on the couch