Я сумую, коли на вулиці темно.

It can mean either, depending on context.

In this sentence, сумую means something like I feel sad / I get gloomy / I feel melancholy.

The verb сумувати is also very commonly used for missing someone, especially with за:

• Я сумую за тобою = I miss you

So the context tells you which meaning is intended. Since this sentence talks about it being dark outside, the emotional meaning I feel sad is the natural one.

Сумую is the 1st person singular present tense form of сумувати.

So:

• я сумую = I feel sad / I miss
• ти сумуєш = you feel sad / you miss
• він/вона сумує = he/she feels sad / misses

The ending -ю tells you the subject is I, which is one reason Ukrainian can often leave out Я.

Yes, absolutely.

Ukrainian often drops subject pronouns when they are clear from the verb ending. Since сумую already shows I, Я is not required.

• Я сумую, коли на вулиці темно = perfectly correct
• Сумую, коли на вулиці темно = also perfectly correct

Including Я can add a little emphasis or make the subject more explicit.

Because коли на вулиці темно is a subordinate clause introduced by коли.

Ukrainian normally separates this kind of clause with a comma:

• Я сумую, коли на вулиці темно.
• Коли на вулиці темно, я сумую.

This is very standard punctuation.

Here коли means when and introduces a time clause.

So it connects the two ideas:

• Я сумую = I feel sad
• коли на вулиці темно = when it is dark outside

Also, коли can be used as a question word:

• Коли ти прийдеш? = When will you come?

So it can be either:

• a question word = when?
• a conjunction = when

In your sentence, it is a conjunction.

Literally, yes, на вулиці is on the street. But in many contexts it is used idiomatically to mean outside / outdoors.

So in this sentence, на вулиці темно means it is dark outside.

Grammatically:

• вулиця = street
• after на in a location meaning, Ukrainian often uses the locative case
• вулиця becomes вулиці

So:

• на вулиці = on the street / outside

Because in Ukrainian, the present-tense verb to be is usually omitted.

English says:

• it is dark outside

Ukrainian normally just says:

• на вулиці темно

There is no need to add a word for is in the present tense. This is one of the most common differences from English.

Because темно is the form used to describe a general state or environment: it is dark.

By contrast:

• темний = dark, masculine adjective
• темна = dark, feminine adjective
• темне = dark, neuter adjective

Those adjective forms must describe a noun:

• темна кімната = a dark room
• темний ліс = a dark forest

But in на вулиці темно, there is no noun being described. The sentence is about the overall condition outside, so Ukrainian uses темно.

Yes.

It is a normal impersonal sentence in Ukrainian, meaning It is dark outside.

Ukrainian often expresses weather, atmosphere, and general conditions this way:

• Темно. = It is dark.
• Холодно. = It is cold.
• Тепло. = It is warm.
• На вулиці тихо. = It is quiet outside.

So на вулиці темно is fully complete and natural.

Yes, you can say that too.

Ukrainian word order is fairly flexible, and different orders can sound slightly different in emphasis.

These are all possible:

• Я сумую, коли на вулиці темно.
• Я сумую, коли темно на вулиці.
• Коли на вулиці темно, я сумую.

The version you were given is very natural and neutral. Moving темно or putting the коли clause first changes emphasis a bit, but not the core meaning.

Сумую is present tense, but like the English present tense, it can refer to:

• what is happening now
• a usual reaction
• a general truth about your feelings

So this sentence could mean:

• I feel sad when it gets dark outside
• I tend to feel sad whenever it is dark outside

It does not have to mean only this exact moment.

Yes, and that is also very natural.

There is a small difference in style:

• Я сумую = I feel sad / I become sad / I am in a melancholy state
• Мені сумно = I am sad / I feel sad, literally to me it is sad

Both are correct, but they are built differently:

• Я сумую uses a verb
• Мені сумно uses a dative construction plus a state word

So:

• Я сумую, коли на вулиці темно
• Мені сумно, коли на вулиці темно

Both sound natural, just with slightly different structure and feel.