Je regarde l’heure sur ma montre avant de partir.

Because heure begins with a silent h in French. With nouns that start with a vowel sound or silent h, le or la contracts to l’.

So:

• la montre
• l’heure

This is the same pattern as:

• l’école
• l’homme
• l’amie

Because the verb has to be conjugated.

The infinitive is regarder = to look at / to watch.
With je, in the present tense, it becomes je regarde = I look / I am looking.

A quick pattern:

• je regarde
• tu regardes
• il / elle regarde
• nous regardons
• vous regardez
• ils / elles regardent

So in this sentence, je regarde is the correct present-tense form.

Yes. Regarder l’heure literally means to look at the time, but in natural English it often corresponds to to check the time.

French often uses regarder in places where English might prefer check.

So:

• Je regarde l’heure = I look at the time / I check the time

You could also hear voir l’heure, but regarder l’heure is very natural for the action of looking at your watch or clock to know the time.

Sur means on. The phrase sur ma montre literally means on my watch, meaning the time as shown on the watch.

So the idea is:

• Je regarde l’heure sur ma montre
  = I look at the time on my watch

French often uses sur for information displayed on a surface or screen, or something visible on an object.

For example:

• sur mon téléphone = on my phone
• sur l’écran = on the screen

Because montre is a feminine noun: une montre.

So the correct possessive adjective is:

• ma montre = my watch

Compare:

• mon livre = my book (masculine)
• ma montre = my watch (feminine)

One important extra point: sometimes French uses mon before a feminine noun that starts with a vowel sound, but that does not apply here because montre starts with a pronounced m sound.

Because after avant, French normally uses de before an infinitive.

So the pattern is:

• avant de + infinitive

Examples:

• avant de partir = before leaving / before I leave
• avant de manger = before eating
• avant de dormir = before sleeping

So avant partir is not correct here.

Because avant de is followed by an infinitive when the subject is the same as the one in the main clause.

In this sentence, the person who looks at the time and the person who leaves are both je.

So French uses:

• Je regarde l’heure avant de partir.
• Literally: I look at the time before leaving.

If the subject changes, French usually uses a full clause instead:

• Je regarde l’heure avant qu’il parte.
  = I look at the time before he leaves.

So the infinitive partir works because the subject stays the same.

Yes, you could, but it means something slightly less specific.

• Je regarde ma montre = I look at my watch
• Je regarde l’heure sur ma montre = I look at the time on my watch

The full sentence makes it clear that you are checking the time, not just looking at the watch itself.

So both are possible, but Je regarde l’heure sur ma montre is more precise.

Because heure is not something you possess in the same way as the watch. The sentence is talking about the time, in a general sense, and then specifying where you are looking at it: on my watch.

So French says:

• l’heure = the time
• sur ma montre = on my watch

This is similar to English:

• I check the time on my watch, not usually I check my time on my watch

Literally, it means before to leave, but that is not natural English. The natural English translation is:

• before leaving
• before I leave

This is a very common difference between French and English. French often uses:

• avant de + infinitive

while English usually prefers:

• before + -ing
• or before + subject + verb

So:

• avant de partir = before leaving / before I leave