Je dois partir tôt demain matin pour prendre le train.

Dois is the je form of the verb devoir in the present tense.

In French, devoir + infinitive means to have to or must:

• je dois partir = I have to leave
• tu dois attendre = you have to wait

So in this sentence, je dois shows obligation.

After devoir, the next verb stays in the infinitive.

So French works like this:

• je dois partir
• nous devons finir
• elle doit venir

Only devoir is conjugated here. Partir stays in its base form because it depends on dois.

French often uses the present tense for a future action when the time is already clear from the context.

Here, demain matin already tells you the action is in the future, so je dois partir sounds completely natural.

French could also use a future form in some situations, but the present is very common for planned or expected future actions, especially in everyday speech.

Demain matin is a fixed time expression meaning tomorrow morning.

French does not need a word like in here. English says tomorrow morning, and French similarly just says:

• demain matin = tomorrow morning
• demain soir = tomorrow evening
• lundi matin = Monday morning

So this is simply the normal French way to express that time.

Because tôt is closely linked to partir.

The idea is:

• partir tôt = to leave early

Then demain matin adds the time frame.

So partir tôt demain matin sounds natural because French keeps the short adverb tôt close to the verb it modifies. In other words, the sentence first gives the action and manner, then the time.

Pour + infinitive expresses purpose. It means to, in order to, or so as to.

So:

• pour prendre le train = to catch/take the train

It answers the question why?

Why does the speaker have to leave early?
Pour prendre le train.

This is a very common French structure:

• Je viens pour aider.
• Elle étudie pour réussir.

In French, prendre le train is the normal expression for taking/catching the train.

French often uses a definite article where English may sound more general. So le train here does not necessarily mean one very specific train in the way an English learner might first expect. It is just the usual idiomatic expression.

If you said un train, it would sound more like a train / one train, which is less natural in this context unless you want to emphasize that it is one train among others.

Yes.

French prendre is broader than English take in some cases. With transport, it can cover ideas like:

• taking the train as a means of travel
• catching the train
• boarding the train

The exact English translation depends on context. In this sentence, because the speaker has to leave early, English would often say catch the train, but French still very naturally uses prendre le train.

Yes, you could.

Attraper le train puts more focus on catching it in time.
Prendre le train is more neutral and more common as the default expression.

So:

• pour prendre le train = neutral, standard
• pour attraper le train = stresses not missing it

Both are possible, but prendre is the safer everyday choice.