J’étais en train de me démaquiller quand tu as frappé à la porte.

Because French usually uses the imparfait for a background action that was already in progress, and the passé composé for a single completed event that happened during it.

So in this sentence:

• J’étais en train de me démaquiller = the ongoing background action
• tu as frappé à la porte = the completed event that interrupted it

This is a very common pattern in French: I was doing X when Y happened
→ Je faisais X quand Y est arrivé

Être en train de + infinitive means to be in the middle of doing something or to be in the process of doing something.

So:

• J’étais en train de me démaquiller
  means something like
• I was in the middle of removing my makeup

It adds emphasis that the action was actively happening at that moment.

Without it, you could also say:

• Je me démaquillais quand tu as frappé à la porte.

That is also correct, but en train de makes the ongoing action feel a bit more vivid or explicit.

The verb here is se démaquiller, which is a reflexive verb. It means to remove one’s makeup.

The me refers back to the subject je. So literally it is something like:

• I was removing makeup from myself

In natural English, of course, we just say I was removing my makeup.

With reflexive verbs, the reflexive pronoun changes with the subject:

• je me démaquille
• tu te démaquilles
• il/elle se démaquille
• nous nous démaquillons
• vous vous démaquillez
• ils/elles se démaquillent

Because after en train de, French uses an infinitive, and reflexive pronouns go directly before that infinitive.

So the structure is:

• être en train de + reflexive pronoun + infinitive

That gives:

• être en train de me démaquiller

Not:

• être en train de démaquiller me

The same pattern appears with other reflexive verbs:

• Je suis en train de me préparer.
• Elle est en train de se laver les cheveux.

No. It can be either reflexive or non-reflexive.

• démaquiller quelqu’un = to remove someone’s makeup
• se démaquiller = to remove your own makeup

Examples:

• Le maquilleur démaquille l’acteur.
  The makeup artist removes the actor’s makeup.

• L’acteur se démaquille.
  The actor removes his own makeup.

So in your sentence, me démaquiller shows that the speaker is removing their own makeup.

Because tu as frappé presents the knocking as a single, completed event.

In this sentence, the knocking is the event that happens during the longer ongoing action. French typically uses:

• imparfait for the ongoing action
• passé composé for the interrupting event

So:

• j’étais en train de me démaquiller = ongoing
• tu as frappé = one completed action

If you said tu frappais, it would suggest that you were knocking as an ongoing action too, which changes the picture.

Frapper literally can mean to strike or to hit, but frapper à la porte is the normal expression for to knock on the door.

So:

• Tu as frappé à la porte = You knocked on the door

It is an idiomatic expression. Even though English says on the door, French normally says à la porte.

Because French uses the expression frapper à la porte as a fixed idiomatic phrase.

English says knock on the door, but French says knock at the door in form:

• frapper à la porte

This is one of those places where you should learn the whole expression, rather than translating the preposition word for word.

Yes, absolutely. That sentence is correct.

• Je me démaquillais quand tu as frappé à la porte
• J’étais en train de me démaquiller quand tu as frappé à la porte

Both mean that you were removing your makeup when the other person knocked.

The difference is mainly nuance:

• Je me démaquillais = simple ongoing past action
• J’étais en train de me démaquiller = stronger emphasis on being in the middle of the action right then

So en train de is often used when you want to stress I was right in the middle of it.

Quand means when, but it does not force one specific tense by itself. The tense depends on the relationship between the two actions.

Here:

• the first action is ongoing → imparfait
• the second action happens during it → passé composé

So:

• J’étais en train de me démaquiller quand tu as frappé à la porte.

French often uses this pattern after quand when one past action is in progress and another one occurs.

You put ne ... pas around the conjugated verb, not around the infinitive.

So:

• Je n’étais pas en train de me démaquiller quand tu as frappé à la porte.

That means:

• I wasn’t in the middle of removing my makeup when you knocked at the door.

Notice that me démaquiller stays together after en train de.

A natural spoken question would be:

• Tu as frappé à la porte quand j’étais en train de me démaquiller ?

Or more directly:

• Est-ce que tu as frappé à la porte quand j’étais en train de me démaquiller ?

If you want to keep the original structure and ask about the whole event, you could also ask:

• Qu’est-ce qui s’est passé quand j’étais en train de me démaquiller ?

The grammar of the original clause stays the same: j’étais en train de me démaquiller.