Learn Hardcore Ukrainian: Тобі треба одягнути теплий светр у лютому. - You need to put on a warm sweater in February.

Because треба is an impersonal modal word (“to need, have to”) and does not have a grammatical subject. The person who needs to do something is expressed in the dative case. So “you” becomes тобі, not ти.

What does треба mean here and how is it used?

Треба means “one needs to,” “it’s necessary to,” or simply “need to.” Key points:

It’s impersonal: there is no subject pronoun like “he” or “she.”
Structure: dative person
- треба
  - infinitive.
Examples: Мені треба читати (I need to read), йому треба спати (he needs to sleep).

Why is the verb одягнути in the infinitive, and why perfective?

The infinitive is required after треба: you always say треба + що зробити? (what to do?).
Одягнути is the perfective form, signaling a single, complete action: “to put on” (once).
If you wanted to express a habitual or ongoing action, you would use the imperfective одягати (e.g. треба одягати шарф щодня – you need to put on a scarf every day).

What’s the difference between одягнути and надягнути?

Traditionally, надягнути is “to put on (clothes) oneself” or “to put garments onto someone.”
Одягнути in strict normative Ukrainian often means “to dress someone” or “to clothe.”
However, in everyday speech many speakers use одягнути interchangeably with надягнути for “put on.”
If you want to follow prescriptive grammar, use надягнути светр for “put on a sweater.”

Why is теплий светр unchanged (appears like nominative) rather than in the accusative?

Because светр is an inanimate masculine noun. In Ukrainian, inanimate masculine nouns have the same form in the nominative and accusative singular. Adjectives agree, so теплий also looks like the nominative but functions as the accusative here.

Why do we say у лютому instead of в лютому for “in February”?

Both в and у mean “in/into,” but to avoid awkward sound clusters Ukrainian tends to use:

в before words starting with a vowel (e.g. в останній раз),
у before words beginning with a consonant (e.g. у лютому, у березні).

Why is лютому the form after у, and how is it formed?

After the preposition у (meaning “in”), the noun “лютий” (the month February) goes into the prepositional case.
Masculine adjectives in the singular prepositional end in –ому. So лютий → лютому.

Why isn’t there an article like “a” or “the” before “теплий светр”?

Ukrainian does not have definite or indefinite articles (“a,” “an,” “the”). Context alone tells you whether you mean “a warm sweater,” “the warm sweater,” or “warm sweaters” generally.

Can I change the word order in this sentence?

Yes. Ukrainian has relatively free word order. For example:

У лютому тобі треба одягнути теплий светр.
Тобі треба в лютому теплий светр одягнути.
The emphasis shifts slightly, but the meaning stays the same.

How would I make this sentence negative, for example “you don’t need to put on a warm sweater in February”?

Insert не before треба or before the verb depending on nuance:

Тобі не треба одягнути теплий светр у лютому. (You don’t need to do it.)
Or using the imperfective: Тобі не треба одягати теплий светр у лютому.