Коли я смажу овочі, я спочатку додаю цибулю.

Here коли means when and introduces a time clause: When I fry vegetables...

In a sentence like this, it often has a whenever sense too, because the speaker is talking about a usual habit:

Коли я смажу овочі... = When/Whenever I fry vegetables...

So the idea is not one specific moment only, but a repeated situation.

Because the sentence describes a habit or a usual way of cooking, not one single future event.

Ukrainian, like English, often uses the present tense for repeated actions:

Коли я смажу овочі, я спочатку додаю цибулю.
= When I fry vegetables, I first add onion.

If you wanted to talk about one future occasion, you would normally use future forms, for example:

Коли я смажитиму овочі, я спочатку додам цибулю.
= When I fry vegetables, I’ll add onion first.

Yes, one of them can be omitted.

Ukrainian often drops subject pronouns because the verb ending already shows the person:

• смажу = I fry
• додаю = I add

So this is also natural:

Коли я смажу овочі, спочатку додаю цибулю.

Repeating я is not wrong. It can make the sentence feel a bit clearer, more balanced, or slightly more emphatic. In learner-friendly, careful speech, repeating it is very common.

Смажу is the 1st person singular present form of смажити.

So:

• я смажу = I fry / I am frying
• ти смажиш = you fry
• він/вона смажить = he/she fries

A useful thing to remember is that Ukrainian present tense often covers both English meanings:

• I fry
• I am frying

So я смажу овочі can mean either, depending on context.

Овочі is the plural form of овоч (vegetable).

In this sentence it is the direct object of смажу. For inanimate plural nouns, the accusative plural is usually the same as the nominative plural, so:

• nominative: овочі
• accusative: овочі

That is why you see смажу овочі.

Compare that with animate nouns, where the accusative is different:

• Я бачу студентів = I see students

Спочатку means first, at first, or to begin with, depending on context.

In this cooking sentence, the best meaning is:

first / first of all

So:

я спочатку додаю цибулю
= I add onion first

It can move around in the sentence:

• Я спочатку додаю цибулю
• Спочатку я додаю цибулю

Both are natural. The difference is mainly emphasis.

Because додаю is the imperfective form, and it fits a habitual/repeated action.

• додаю = I add / I am adding regularly, as a process or habit
• додам = I will add once, as a completed future action

In this sentence, the speaker is explaining what they usually do while cooking, so додаю is the natural choice.

If it were about one future cooking event, then додам would work:

Коли я смажитиму овочі, я спочатку додам цибулю.

Because it is in the accusative singular.

The dictionary form is цибуля (onion). But here it is the direct object of додаю, so it changes form:

• nominative: цибуля
• accusative: цибулю

This is a very common pattern for feminine nouns ending in -я.

Compare:

• я люблю Марію
• я бачу землю
• я додаю цибулю

So додаю цибулю literally means I add onion.

Yes. The comma is standard because Коли я смажу овочі is a subordinate clause, and it is separated from the main clause:

Коли я смажу овочі, я спочатку додаю цибулю.

If the order is reversed, the comma still appears:

Я спочатку додаю цибулю, коли смажу овочі.

So the comma is not optional here in normal written Ukrainian.

The word order can change quite a bit.

Ukrainian word order is more flexible than English because the endings show grammatical roles. So several versions are possible:

• Коли я смажу овочі, я спочатку додаю цибулю.
• Коли я смажу овочі, спочатку додаю цибулю.
• Спочатку я додаю цибулю, коли смажу овочі.

The version you have is very clear and natural. Different word orders mainly change focus or emphasis, not the core meaning.

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

That means:

• овочі can mean vegetables or the vegetables
• цибулю can mean onion, an onion, the onion, or even some onion, depending on context

In this sentence, the most natural English translation is the general cooking meaning:

When I fry vegetables, I first add onion.

So the exact article choice comes from the situation, not from a separate word in Ukrainian.