Коли в кімнаті багато пилу, я починаю чхати.

Here коли means when in the sense of whenever or in situations where.

So the sentence describes a regular pattern:

• Коли в кімнаті багато пилу, я починаю чхати.
• When/Whenever there is a lot of dust in the room, I start sneezing.

If you wanted a more conditional if, Ukrainian would usually use якщо instead.

In Ukrainian, the verb to be is often omitted in the present tense.

So instead of saying something like In the room there is a lot of dust, Ukrainian simply says:

• в кімнаті багато пилу
• literally: in the room much dust

This is completely normal. You may sometimes see є, but in simple present-tense statements it is very often left out.

Because after в or у meaning in, Ukrainian uses the locative case for location.

The basic form is:

• кімната = room

But after в for location, it becomes:

• в кімнаті = in the room

So:

• nominative: кімната
• locative: в кімнаті

Because багато normally requires the genitive case.

The noun is:

• пил = dust

Its genitive singular form here is:

• пилу

So:

• багато пилу = a lot of dust

This is a very common pattern in Ukrainian:

• багато часу = a lot of time
• багато води = a lot of water
• багато людей = a lot of people

Because dust is an uncountable or mass noun here.

In English, you also usually say a lot of dust, not many dusts. Ukrainian works similarly. Since пил refers to a substance rather than separate countable items, the singular is natural:

• багато пилу

If you were talking about different kinds of dust in some specialized context, other forms might appear, but in ordinary speech this singular genitive form is the normal one.

There is a difference in nuance:

• я чхаю = I sneeze / I am sneezing
• я починаю чхати = I start sneezing

So починаю чхати emphasizes the beginning of the reaction. It suggests that the dust triggers the sneezing.

If you said:

• Коли в кімнаті багато пилу, я чхаю

that would also be possible, but it means more simply I sneeze when there’s a lot of dust in the room, without focusing on the start of it.

Because after починати (to begin/start), Ukrainian normally uses an infinitive to describe the action that begins.

So:

• починаю = I begin / I start
• чхати = to sneeze

Together:

• починаю чхати = I start sneezing

This is similar to English start + -ing or start + infinitive.

Other examples:

• починаю працювати = I start working
• починаю читати = I start reading

Починаю is the present-tense form of the imperfective verb починати. It fits well because the whole sentence describes a habitual or repeated situation:

• Whenever there is a lot of dust, I start sneezing.

If you said почну, that would usually point to one future event:

• Коли в кімнаті буде багато пилу, я почну чхати.
• When there is a lot of dust in the room, I will start sneezing.

So починаю is better for a general repeated fact.

Because Коли в кімнаті багато пилу is a subordinate clause introduced by коли.

In Ukrainian, subordinate clauses are separated by a comma:

• Коли в кімнаті багато пилу, я починаю чхати.

This is similar to English, where a comma is also often used when a when-clause comes first.

Yes. Ukrainian word order is flexible.

You can also say:

• Я починаю чхати, коли в кімнаті багато пилу.

This means the same thing. The difference is mainly in emphasis:

• Коли в кімнаті багато пилу, ... puts the situation first
• Я починаю чхати, коли ... puts I start sneezing first

Both are natural.

Yes, both are possible:

• в кімнаті
• у кімнаті

Ukrainian often alternates в and у for ease of pronunciation and rhythm. This is a common stylistic and phonetic feature.

In this sentence, в кімнаті sounds perfectly natural. У кімнаті would also be correct.

In this sentence, it most naturally describes a repeated pattern or general truth:

• Whenever there is a lot of dust in the room, I start sneezing.

Even though коли can refer to a single time, the present-tense structure here makes it sound habitual.

If you wanted to clearly refer to one future occasion, you would usually change the verb forms, for example:

• Коли в кімнаті буде багато пилу, я почну чхати.

That sounds more like one future case.

Yes, many English speakers notice that чхати begins with an unusual cluster.

A rough guide:

• ч sounds like ch in church
• х is a throaty sound, like the ch in German Bach or Scottish loch
• чхати is roughly ch-khA-ty

You do not need to insert a vowel between ч and х. Ukrainian allows consonant clusters that may feel unusual to English speakers.