Я чекаю друга біля входу, але не бачу його.

Чекаю is the 1st person singular present tense of the imperfective verb чекати.

So it means:

• I wait
• I am waiting

Ukrainian does not have a separate present progressive form like English am waiting, so the simple present does both jobs.

Because it is the object form of the noun after the verb чекаю.

With masculine animate nouns in the singular, the accusative/object form usually looks like the genitive form. So:

• друг = friend, as the basic dictionary form
• друга = friend, as the object form here

This is a very common pattern in Ukrainian.

Not in this sentence.

Друга can also be the feminine form of the adjective другий meaning second, as in друга книга = the second book.
But here it comes after чекаю and clearly functions as a noun meaning friend in its object form.

Also, female friend is подруга, not друга.

Because біля is a preposition, and it requires the genitive case.

So:

• вхід = entrance
• входу = of the entrance / entrance in the genitive form

That is why you get біля входу = near the entrance.

Біля usually means near, by, or next to.

In this sentence, біля входу is most naturally near the entrance or by the entrance.
It does not usually mean exactly inside or at in the broader English sense; it suggests physical closeness.

Because він means he, the subject form.
Here the word means him, the object form, so Ukrainian uses його.

Compare:

• Він тут. = He is here.
• Я бачу його. = I see him.

So after бачу, you need його.

Because Ukrainian does not form the present progressive the way English does.

English:

• I wait
• I am waiting

Ukrainian:

• Я чекаю for both meanings

The context tells you whether English should sound more like I wait or I am waiting.

Because with verbs, не is normally written as a separate word.

Examples:

• не бачу = I do not see
• не знаю = I do not know
• не хочу = I do not want

So не бачу is the standard spelling.

Because але joins two clauses:

• Я чекаю друга біля входу
• не бачу його

In Ukrainian, a comma is normally used before але when it connects clauses like this.
So the comma here is standard punctuation.

Yes. Ukrainian word order is fairly flexible because case endings show who is doing what.

The given sentence has a neutral, natural order:

• Я чекаю друга біля входу, але не бачу його.

But other orders are possible for emphasis, for example:

• Біля входу я чекаю друга, але не бачу його.
  This emphasizes near the entrance.
• Я його не бачу.
  This puts more focus on him.

So word order in Ukrainian often affects emphasis more than basic grammar.

Because Ukrainian has no articles like English a/an/the.

So вхід / входу can mean:

• an entrance
• the entrance
• just entrance

The exact meaning is understood from context. In this sentence, English naturally translates it as the entrance, but Ukrainian does not need a separate word for that.