To the answer « Ces hommes parlent sans cesse. »

One of the possible questions could be:

Quelles sont les personnes qui parlent sans cesse ?

A full sentence as an answer disguises what the question was. There’s no way to know which part of the sentence was asked for, except for emphasis or tone. I’ll mark emphasis in bold.

Asking about the subject

Answer: Ces hommes parlent sans cesse.

Question: Qui parle sans cesse ?

When you’re asking about the subject, or who’s doing the action, you have two options: qui if you expect that it’s a person, or qu’est-ce qui if you’re not sure. This goes in the normal sentence order. Don’t forget to make the verb agree with this question word by using the 3rd-person singular ("parle").

As Ced showed, there are other options depending on what you already know about the subject. His translates to "which people", implying that the asker knows it’s a group of people that are talking.

Asking about the verb

Answer: Ces hommes parlent sans cesse.

Question: Qu’est-ce que ces hommes font sans cesse ?

When you’re asking about the verb, or what action it is, you add qu’est-ce que at the beginning, where questions words tend to go, and replace the verb with faire in the same conjugation.

You also have the less formal option of « Ces hommes font quoi sans cesse ? » This is the equivalent of asking "These men are doing what endlessly?" — okay for conversation, a little rough for writing.

Finally, you can invert, which is more formal but you have to move things around carefully, so you might want to avoid it as a beginner. « Que font-ils sans cesse, ces hommes ? »

(There are even further variations possible; see Goat’s comment below.)

Asking about the adverb

Answer: Ces hommes parlent sans cesse.

Question: Comment est-ce que ces hommes parlent ?

When you’re asking about the adverb, or how they’re doing the action, you replace sans cesse with comment, add est-ce que, and move it to the beginning,.

Similarly to the previous one, you can also go less formal and change the adverb in-place: « Ces hommes parlent comment ? »

Or you can go more formal and invert: « Comment ces hommes parlent-ils ? »

Asking about the object

Your example sentence doesn’t include an object, so this is just for completeness.

Answer: Ces hommes parlent sans cesse de la politique.

Question: De quoi est-ce que ces hommes parlent sans cesse ?

To ask about the object, or the topic they’re talking about, replace it with qu’est-ce que or, if it follows a preposition like de, replace it with quoi. Then move this chunk to the beginning, add est-ce que, and you’re golden.

Similar notes on informal version: « Ces hommes parlent sans cesse de quoi ? »

And on inversion: « De quoi ces hommes parlent-ils sans cesse ? »

No, it’s not a proper question, it’s not correct.

In your question you have two pronouns, qui and ils. parler is intransitive so you can use have one.

Qui parle sans cesse ? – Who‘s talking continuously? (this sentence works)

Parlent-ils sans cesse ? – Are they talking continuously? (this one doesn’t, it’s a yes/no question)

With a transitive verb if you have two pronouns, one of them acts as a complement:

Qui suivent-ils ? – Who are they following?