Його щока червона.

In Ukrainian, the present tense of to be is usually omitted.

So Його щока червона literally looks like His cheek red, but it naturally means His cheek is red.

You may see є in some contexts, but in everyday speech it is normally left out here.

Here його means his.

It comes from the pronoun він = he, and before a noun it often works as a possessive:

• його щока = his cheek
• його рука = his hand

By itself, його can also mean him in some contexts, but in this sentence, because it comes directly before a noun, it is understood as his.

Because щока is a feminine noun, and the adjective has to agree with it.

So:

• masculine: червоний
• feminine: червона
• neuter: червоне
• plural: червоні

Since щока is feminine singular, the correct form is червона.

Щока is in the nominative singular.

It is the subject of the sentence: the cheek is what is red.

The adjective червона also matches it in feminine singular nominative.

This is a very important difference:

• Його щока червона = His cheek is red
• його червона щока = his red cheek

In other words:

• noun + adjective at the end can form a full sentence
• adjective before the noun usually just describes the noun inside a noun phrase

So the word order changes the function.

Because червона is not simply describing the noun inside a phrase like red cheek. It is the main predicate of the sentence: the cheek is red.

Compare:

• червона щока = a red cheek
• щока червона = the cheek is red

So the position helps show whether you have a phrase or a complete sentence.

A simple learner-friendly approximation is:

yo-HO shcho-KA cher-VO-na

A few useful notes:

• його: the г sounds like a soft voiced h, not a hard English g
• щ is usually pronounced roughly like shch
• the stress is:
  • його́
  • щока́
  • черво́на

Yes, Ukrainian word order is flexible, but the given sentence is a normal, neutral way to say it:

• Його щока червона.

Other orders are possible, but they can sound more emphatic, literary, or context-dependent.

You may also hear:

• У нього щока червона.

That is another natural way to express roughly the same idea, closer to He has a red cheek or His cheek is red, depending on context.

Because Ukrainian has no articles.

So щока can mean a cheek or the cheek, depending on context. In this sentence, English naturally uses his cheek, so no separate article is needed in translation.

Then you would use the plural:

• Його щоки червоні. = His cheeks are red.

Notice both words change:

• щока → щоки
• червона → червоні

That is because the adjective must agree with the noun in number as well as gender.