Сукня їй пасує, тоді як куртка не пасує.

Їй means to her / for her and it is in the dative case.

The verb пасувати in the sense of to suit someone / to look good on someone normally works like this:

• [thing] пасує [person in dative]

So:

• Сукня їй пасує = The dress suits her
• literally: The dress is suitable to her

That is why Ukrainian uses їй rather than a direct-object form.

Because їй and її are different cases of вона (she).

• її = her as a direct object, or her in the possessive sense
• їй = to her / for her (dative)

With пасувати meaning to suit someone, Ukrainian uses the dative, so їй is correct:

• Сукня їй пасує.

Not:

• Сукня її пасує. ❌

Here пасує means something like:

• suits
• looks good on
• is becoming to
• sometimes goes well with

So Сукня їй пасує means the dress is flattering on her or is a good match for her appearance/style.

This verb is often used for clothing, colors, hairstyles, and similar things.

Because in both parts the subject is a singular noun:

• Сукня = dress, singular
• куртка = jacket, singular

So the verb is 3rd person singular:

• сукня пасує
• куртка не пасує

If the subject were plural, the verb would also be plural:

• Сукні їй пасують. = The dresses suit her.

It can be omitted because it is understood from the first part.

Full version:

• Сукня їй пасує, тоді як куртка їй не пасує.

But repeating їй is unnecessary, so Ukrainian often leaves it out in the second clause:

• Сукня їй пасує, тоді як куртка не пасує.

English does the same sometimes:

• The dress suits her, whereas the jacket doesn’t.

Тоді як means whereas, while, or but by contrast.

It introduces a contrast between two statements:

• Сукня їй пасує, тоді як куртка не пасує.
• The dress suits her, whereas the jacket doesn’t.

It is a bit more explicit and contrastive than simply using і (and).

Yes, often you can.

For example:

• Сукня їй пасує, але куртка не пасує.

This means:

• The dress suits her, but the jacket doesn’t.

The difference is mainly in nuance:

• але = simple but
• тоді як = whereas / in contrast, a slightly more formal or explicitly contrasting connector

Both are natural, but тоді як highlights the comparison more strongly.

Ukrainian has no articles like English a/an/the.

So:

• сукня can mean a dress or the dress
• куртка can mean a jacket or the jacket

You understand which one is meant from the context.

That is completely normal in Ukrainian.

They are in the nominative singular, because they are the subjects of the verb.

• сукня = nominative singular
• куртка = nominative singular

In this sentence, the structure is:

• Сукня = subject
• їй = indirect object / dative experiencer
• пасує = verb

Yes. Ukrainian word order is more flexible than English.

The neutral version here is:

• Сукня їй пасує, тоді як куртка не пасує.

But you could also hear:

• Їй пасує сукня, тоді як куртка не пасує.
• Сукня пасує їй, тоді як куртка — ні.

The basic meaning stays the same, but the emphasis changes.

For example:

• Їй пасує сукня puts a bit more focus on her
• Сукня пасує їй can sound more emphatic or contrastive

No. It is very often used with clothes, colors, hairstyles, and appearance, but it can also be used more broadly for things that fit, match, or suit.

For example, it can refer to:

• clothes suiting a person
• a color suiting someone
• one thing matching another

Still, in everyday learner contexts, you will most commonly meet it with appearance and clothing.

In Ukrainian, не is usually written separately from verbs.

So:

• пасує = suits
• не пасує = does not suit

That is the normal spelling pattern:

• вона знає = she knows
• вона не знає = she does not know

Yes. Ukrainian often avoids repeating obvious information.

For example, you may also hear:

• Сукня їй пасує, а куртка — ні.

Literally:

• The dress suits her, and/as for the jacket — no.

This is a very natural way to avoid repeating пасує. The omitted verb is understood from context.