У супі було забагато цибулі, але сестрі все одно сподобалося.

Both у and в mean in. Ukrainian often switches between them for smoother pronunciation, so у супі and в супі are both possible.

супі is the locative singular of суп. After у/в when you mean location, Ukrainian normally uses the locative case.

Here було is the past tense of бути used in an impersonal way, meaning something like there was.

Ukrainian often uses neuter singular було in sentences about existence or amount when there is no normal nominative subject. So У супі було забагато цибулі is literally close to In the soup there was too much onion.

Yes, забагато is one word.

It means too much or too many. Compare:

• багато = a lot / many
• забагато = too much / too many

Because words of quantity such as багато, мало, достатньо, and забагато usually require the following noun to be in the genitive.

So забагато цибулі means too much onion.
цибулі is the genitive singular form of цибуля.

Because подобатися / сподобатися works like to be pleasing to someone, not like English to like.

So the person who likes something is put in the dative:

• сестрі = to the sister / to my sister / to his sister, depending on context

A very literal version would be: це сподобалося сестрі = it was pleasing to my sister

сподобатися is perfective, while подобатися is imperfective.

Here the sentence describes a single completed reaction: despite the too much onion, she liked it on that occasion. That is why сподобалося is natural.

подобалося would suggest something more ongoing, repeated, or descriptive, like was pleasing or used to like.

Because the thing that was liked is not repeated as an explicit noun. Ukrainian often uses neuter singular here as if there were an understood це meaning it / that.

So але сестрі все одно сподобалося is basically: but my sister still liked it

You could also say але сестрі це все одно сподобалося if you wanted to make the it explicit.

In this sentence, все одно means still, anyway, or all the same.

It shows contrast: there was too much onion, but that did not change the final result. She liked it despite that.

Because Ukrainian often omits pronouns when they are clear from context.

English usually needs it, but Ukrainian can simply say сестрі все одно сподобалося. The listener understands that the missing idea is it / that / the soup.

Ukrainian has no articles, so there is no direct equivalent of the or a.

Also, possessive words such as моя are used only when needed for clarity or emphasis. So сестрі on its own can be understood from context as to the sister, to my sister, to her sister, and so on.

Yes. Ukrainian word order is fairly flexible because case endings show what each word is doing.

The given order sounds natural and highlights the contrast well: У супі було забагато цибулі, але сестрі все одно сподобалося.

But other orders are possible, for example: Але сестрі все одно сподобалося.

The meaning stays basically the same, though the emphasis can shift a little.