Я сиджу на стільці.

A literal breakdown is:

• Я = I
• сиджу = am sitting / sit
• на = on
• стільці = on a chair / on the chair depending on context

So the sentence is literally I sit/am sitting on chair, but natural English adds an article: I am sitting on a chair or I am sitting on the chair.

Because Ukrainian does not form the present continuous the way English does.

In English, I am sitting has:

• the verb to be = am
• the main verb in -ing form = sitting

In Ukrainian, one verb form does the job by itself:

• сиджу = I sit / I am sitting

So сиджу already includes the idea of I plus present tense. No extra word for am is needed.

It can mean either. Ukrainian has no articles like a/an/the.

So на стільці can mean:

• on a chair
• on the chair

Context tells you which one is meant.

For example:

• if you are introducing it for the first time, English may use a chair
• if both speakers know which chair, English may use the chair

Ukrainian leaves that unstated unless something else makes it specific.

Because after на in this sentence, the noun is in the locative case, since it shows location.

The dictionary form is:

• стілець = chair

But after на when you mean on the chair / on a chair, it changes to:

• на стільці

So:

• стілець = nominative, basic form
• стільці = locative singular

This is a very common pattern in Ukrainian: nouns change form depending on their role in the sentence.

Because with a chair, Ukrainian normally treats it as a surface you sit on.

So:

• на стільці = on a chair
• на ліжку = on the bed
• на дивані = on the sofa

Using в would suggest being inside something, which does not fit an ordinary chair.

A useful comparison:

• на стільці = on a chair
• в кріслі = in an armchair

So the preposition depends on how the object is conceptualized.

Yes, very often.

You can simply say:

• Сиджу на стільці.

That still means I am sitting on a chair, because the verb form сиджу already shows first person singular.

Including я is usually for:

• emphasis
• contrast
• clarity

For example:

• Я сиджу на стільці, а він стоїть. = I am sitting on a chair, and he is standing.

Сиджу is the 1st person singular present form of сидіти.

So:

• сидіти = to sit / to be sitting
• я сиджу = I sit / I am sitting

The form changes quite a bit, so it is best learned as part of the verb’s present-tense pattern:

• я сиджу = I sit
• ти сидиш = you sit
• він/вона сидить = he/she sits
• ми сидимо = we sit
• ви сидите = you sit
• вони сидять = they sit

For an English speaker, сиджу may look irregular at first, but it is a normal verb form that just needs memorizing.

This is an important difference.

• сидіти / сиджу = to be sitting, the state
• сісти = to sit down, a completed action
• сідати = to sit down, often repeated or in process

So:

• Я сиджу на стільці. = I am sitting on a chair.
• Я сідаю на стілець. = I am sitting down on the chair.
• Я сів на стілець. = I sat down on the chair.

In other words:

• сиджу describes the position you are already in
• сісти / сідати describe the movement into that position

Because Ukrainian changes case depending on whether you mean location or movement toward something.

• Я сиджу на стільці. = I am sitting on the chair
  
  • this is location, so locative
• Я сідаю на стілець. = I am sitting down onto the chair
  
  • this is direction/target, so accusative

So:

• на стільці = on the chair, already there
• на стілець = onto the chair, movement toward it

This location-vs-motion contrast is very common in Ukrainian.

Yes. Ukrainian word order is more flexible than English.

The neutral order here is:

• Я сиджу на стільці.

But you can also hear:

• Сиджу на стільці.
• На стільці я сиджу.

These all can be correct, but the emphasis changes:

• Я сиджу на стільці. = neutral
• Сиджу на стільці. = natural when the subject is obvious
• На стільці я сиджу. = emphasis on where I am sitting

So word order in Ukrainian often reflects focus more than strict grammar.

A rough pronunciation guide is:

• Я ≈ ya
• сиджу ≈ syd-ZHOO
• на ≈ na
• стільці ≈ stil-TSI

A few useful details:

• ж sounds like the s in measure
• дж here is pronounced together, like the j sound in judge
• і sounds like ee
• ль is a soft l
• ц sounds like ts

The stress is usually:

• сиджу́
• стільці́

So the sentence sounds roughly like:

• ya syd-ZHOO na stil-TSI

It can mean either, depending on context.

Ukrainian present tense often covers both:

• an action happening now
• a general or habitual action

So Я сиджу на стільці can mean:

• I am sitting on a chair right now
• I sit on a chair in some general sense

Usually, without extra context, people are likely to understand it as a present situation.

If you want to make it clearly habitual, you can add a word like:

• Зазвичай я сиджу на стільці. = I usually sit on a chair.

If you want to make it clearly right now, context often does that naturally.