Папа почувствовал запах газа и сразу проверил, нет ли утечки.

Yes. Here Папа is capitalized because it is the first word of the sentence.

In Russian, папа is normally written with a lowercase letter, just like dad in English when it is used as a common noun. You would only capitalize it at the start of a sentence, or sometimes in very personal writing styles.

So in the middle of a sentence, you would usually write:

Мой папа почувствовал запах газа.

Почувствовал is a perfective past-tense verb. It means something like noticed / felt / sensed at a particular moment.

That fits this sentence well, because the idea is:

• he suddenly detected the smell
• then he immediately checked for a leak

If you used чувствовал, it would usually suggest an ongoing state, more like was smelling / was feeling something over a period of time.

So:

• почувствовал запах газа = he noticed the smell of gas
• чувствовал запах газа = he was smelling gas / he could smell gas

Газа is in the genitive case.

The noun запах often takes another noun in the genitive to show the smell of something:

• запах кофе = the smell of coffee
• запах дыма = the smell of smoke
• запах газа = the smell of gas

So this is a very common Russian pattern:

[noun] + genitive = [noun] of [something]

Russian does not have articles like a and the.

So запах газа can mean:

• a smell of gas
• the smell of gas
• simply gas smell

The exact meaning depends on context. Russian speakers do not usually need a separate word for this.

Сразу means right away, immediately, or at once.

In this sentence, it modifies проверил:

и сразу проверил = and immediately checked

Russian word order is flexible, so you could also see:

• Папа сразу проверил, нет ли утечки.
• Папа почувствовал запах газа и проверил сразу, нет ли утечки.

But и сразу проверил sounds very natural here because it emphasizes the quick reaction after noticing the smell.

Again, this is about aspect.

Проверил is perfective, showing a completed action: he checked.

That works well because the sentence tells a short sequence of completed events:

1. he noticed the smell
2. he checked whether there was a leak

If you said проверял, that would suggest an ongoing or repeated process, more like was checking.

Compare:

• проверил = checked, completed the check
• проверял = was checking / used to check

Literally, it is something like:

whether there is not a leak

But in natural English, it means:

whether there was a leak or if there was a leak

This is a very common Russian pattern after verbs like check, see, find out, ask, etc.:

• проверить, нет ли... = to check whether there is no...
• посмотреть, нет ли... = to see whether there is no...
• спросить, нет ли... = to ask whether there is no...

Russian often uses this negative pattern where English simply says if there is... or whether there is...

Ли is a question particle often used in indirect questions, especially in the sense of whether.

In Russian, ли usually comes right after the word it is attached to.

So:

• нет ли утечки = whether there is no leak
• literally, the uncertainty is attached to нет

You can think of it as:

• нет = there is not
• ли = whether

Together: нет ли = whether there is not / if there is no

This word order is normal in Russian.

Because нет requires the genitive case.

The basic noun is:

• утечка = leak

But after нет, Russian uses the genitive:

• нет утечки = there is no leak

So in нет ли утечки, the noun stays in the genitive singular:

• nominative: утечка
• genitive: утечки

This is one of the most important genitive patterns in Russian.

Because нет ли утечки is a subordinate clause.

The main clause is:

Папа почувствовал запах газа и сразу проверил

Then comes what he checked:

нет ли утечки

Russian normally separates this kind of subordinate clause with a comma.

You can think of it as:

Dad smelled gas and immediately checked, whether there was a leak.

Even though English would usually not put a comma there, Russian does.

Yes, it could, but the nuance is slightly different.

• проверил, нет ли утечки = checked whether there was any leak / checked that there wasn’t a leak
• проверил, есть ли утечка = checked whether there was a leak

Both are possible. In real Russian, нет ли is very common in situations where someone is checking for a possible problem, danger, or unwanted thing.

For example:

• Проверь, нет ли ошибки. = Check if there is a mistake.
• Посмотри, нет ли письма. = See if there is a letter.
• Он проверил, нет ли дыма. = He checked whether there was smoke.

So нет ли утечки sounds especially natural here because a leak is something undesirable that one hopes is absent.

Yes. Папа is masculine, even though it has a noun ending that often looks feminine.

This is a small group of masculine nouns referring to male people, such as:

• папа
• дядя
• дедушка
• мужчина
• юноша

They decline similarly to many feminine-looking nouns, but they are grammatically masculine.

That is why the past-tense verb is:

• почувствовал and проверил for masculine

If the subject were feminine, you would get:

• Мама почувствовала запах газа и сразу проверила, нет ли утечки.

Yes, Russian word order is fairly flexible, though some versions sound more natural than others.

Original: Папа почувствовал запах газа и сразу проверил, нет ли утечки.

Possible alternatives:

• Почувствовав запах газа, папа сразу проверил, нет ли утечки.
• Папа сразу проверил, нет ли утечки, почувствовав запах газа.
• Запах газа папа почувствовал и сразу проверил, нет ли утечки.

The original version is very natural because it presents the events in a clear sequence:

1. he noticed the smell
2. he checked

So while Russian allows variation, the original is probably the best neutral phrasing.