J’étais en train de mesurer le mur quand Marie m’a demandé le mètre.

Because French is describing an action that was ongoing in the past.

Here, j’étais en train de mesurer means I was in the middle of measuring. That calls for the imparfait: j’étais.

• j’étais = I was
• j’ai été = I was / I have been in the sense of a more completed fact or event

In this sentence, the speaker is setting the scene: the measuring was already happening when something else occurred.

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 mesurer = I was in the middle of measuring
• literally, something like I was in the act of measuring

It adds emphasis to the idea that the action was actively going on at that moment.

Without it, you could also say:

• Je mesurais le mur quand Marie m’a demandé le mètre.

That is also correct, but en train de makes the ongoing action feel more vivid and immediate.

Because after être en train de, French uses the infinitive form of the verb.

So the pattern is:

• être en train de faire
• être en train de lire
• être en train de mesurer

That is why you get:

• j’étais en train de mesurer

not a conjugated form after de.

This is a very common French past-tense pattern:

• imparfait for the background / ongoing action
• passé composé for the main event / interruption / completed action

So here:

• J’étais en train de mesurer le mur = background, ongoing action
• quand Marie m’a demandé le mètre = the event that happened during that action

A natural way to think of it is:

• I was measuring the wall
• when Marie asked me for the tape measure

This contrast between imparfait and passé composé is one of the most important uses of the two tenses together.

M’ is the shortened form of me.

Here it means to me or from me, depending on how you translate the sentence naturally in English.

It comes before the verb because French object pronouns usually go before the conjugated verb. In a compound tense like a demandé, the pronoun goes before the auxiliary:

• Marie m’a demandé...
• literally: Marie to-me has asked...

It becomes m’ instead of me because the next word, a, begins with a vowel.

Yes. Demander can mean several related things, including:

• to ask
• to ask for
• to request

In this sentence, it means to ask someone for something.

A useful pattern is:

• demander quelque chose à quelqu’un = to ask someone for something

So:

• Marie m’a demandé le mètre = Marie asked me for the measuring tape / meter stick

It is not necessarily asking a question. It can simply be a request.

Here, le mètre most likely means a measuring tool, not the unit of length.

Depending on context, it can refer to:

• a tape measure
• a meter stick
• a measuring rule

Because the sentence is about measuring a wall, that tool meaning is the natural one.

So although mètre can mean the unit meter, in everyday contexts like this it often means the measuring tape / measure.

French often uses the definite article when the thing is specific in the situation or understood from context.

So here:

• le mur = the wall being measured
• le mètre = the measuring tool being used or known in that situation

English sometimes uses the, and sometimes it might sound more natural to say a, depending on context. French is often a bit more comfortable using le / la / les when the speaker has a particular thing in mind.

So this sentence sounds like both people know which wall and which measuring tool are involved.

Yes, absolutely.

• Je mesurais le mur = I was measuring the wall
• J’étais en train de mesurer le mur = I was in the middle of measuring the wall

Both are correct here.

The difference is mainly nuance:

• je mesurais is simpler and more neutral
• j’étais en train de mesurer emphasizes that the action was actively in progress at that exact moment

So the version with en train de is a little more explicit.

Because demandé is used with avoir, and in this sentence there is no preceding direct object that would trigger agreement.

Also, m’ here is not the direct object of demander. It represents the person being asked, while le mètre is the thing being asked for.

So French keeps:

• Marie m’a demandé le mètre

with demandé unchanged.

This is a more advanced point, but it explains why you do not write an extra e or s on demandé here.