Learn Hardcore French: Je me rendors rapidement après le cauchemar. - I fall back asleep quickly after the nightmare.

se is the reflexive pronoun that changes with the subject:
- je me rends (myself)
- tu te rends (yourself)
- il/elle se rend (himself/herself), etc.

So:

je me rendors = “I go back to sleep (myself)”
Saying je rendors without me is ungrammatical here. Rendormir without se would mean “to make someone fall asleep again,” which is very rare and not what’s happening in this sentence.

What is the difference between je dors, je m’endors, and je me rendors?

These three are related but not interchangeable:

je dors – “I am sleeping / I sleep.”
- Describes the state of being asleep.
je m’endors – “I’m falling asleep / I fall asleep.”
- Describes the process of going from awake to asleep.
je me rendors – “I’m falling asleep again / I go back to sleep.”
- Describes going back to sleep after having woken up.

In the sentence Je me rendors rapidement après le cauchemar, the person was asleep, woke up because of the nightmare, and then falls asleep again.

Is rendors present tense? How is se rendormir conjugated for je?

Yes, rendors is present tense, 1st person singular of se rendormir.

The infinitive is se rendormir. In the present tense:

je me rendors
tu te rendors
il/elle/on se rendort
nous nous rendormons
vous vous rendormez
ils/elles se rendorment

Notice the stem change: rendorm- → rendors / rendor- in some forms, similar to dormir → je dors, nous dormons.

Why is the pronoun before the verb: je me rendors and not je rendors me?

In French, unstressed object pronouns (like me, te, se, nous, vous, le, la, lui, leur, y, en) go before the conjugated verb in simple tenses:

je me rendors
tu te réveilles
il se lève

Putting the pronoun after the verb (je rendors me) is incorrect in normal statements.

In other tenses/structures, the placement changes, for example:

Future simple: je me rendormirai
Near future: je vais me rendormir (before the infinitive)
Negative: je ne me rendors pas (pronoun still before the verb)

Could this sentence be in the past? How would I say “I went back to sleep quickly after the nightmare”?

The given sentence is in the present tense: Je me rendors rapidement après le cauchemar.

To put this in the past (passé composé) you’d say:

Je me suis rendormi rapidement après le cauchemar. (if you are male)
Je me suis rendormie rapidement après le cauchemar. (if you are female)

Notes:

Reflexive verbs in the passé composé use être as the auxiliary: je me suis…
The past participle rendormi agrees in gender and number with the subject when the reflexive pronoun is a direct object: rendormi (m.sg.), rendormie (f.sg.), rendormis (m.pl.), rendormies (f.pl.).

Why is rapidement after the verb? Could I also say je me rendors vite?

Rapidement is an adverb (like “quickly”), and a very common position for adverbs in French is after the conjugated verb:

je mange rapidement – I eat quickly
je me rendors rapidement – I fall back asleep quickly

You can certainly say:

Je me rendors vite après le cauchemar.

Differences in nuance:

rapidement is a bit more neutral/formal.
vite is more colloquial/spoken and slightly shorter/stronger in feel.

Both are perfectly correct here.

Why do we say après le cauchemar and not après un cauchemar or après du cauchemar?

Après is a preposition that means “after,” and it is followed by a noun phrase:

après le cauchemar – after the nightmare
après ce cauchemar – after this nightmare
après un cauchemar – after a nightmare

In your sentence, le cauchemar suggests a specific nightmare the speaker and listener both know about (for example, one that just happened).

We do not say après du cauchemar here:

du would be a partitive article (“some nightmare”), which doesn’t work with après in this meaning. You need a normal article (le, un, ce...) before cauchemar.

Is cauchemar masculine or feminine? How do I know which article to use?

Cauchemar is a masculine noun in French.

That’s why the sentence has:

le cauchemar (the nightmare)
not la cauchemar

Other forms:

un cauchemar – a nightmare
ce cauchemar – this nightmare
mon cauchemar – my nightmare

Unfortunately, grammatical gender usually has to be learned case by case; there’s no rule that all words ending in -mar are masculine, for example. Dictionaries always mark the gender (nm for nom masculin, nf for nom féminin).

Could I say après avoir fait un cauchemar instead of après le cauchemar? What’s the difference?

Yes, you can say:

Je me rendors rapidement après avoir fait un cauchemar.

Both are correct, but the focus is slightly different:

après le cauchemar
- Focuses on the nightmare as an event: after the nightmare is over.
après avoir fait un cauchemar
- Literally “after having had a nightmare / after having done a nightmare.”
- Focuses on the experience of having the nightmare, and it uses a verbal structure (avoir fait) instead of just a noun.

In everyday speech, both are natural, with maybe après un cauchemar or après avoir fait un cauchemar being more common than après le cauchemar if no specific nightmare was mentioned before.

How do you pronounce je me rendors rapidement après le cauchemar? Any silent letters or tricky sounds?

Main points of pronunciation:

je → /ʒə/ (the j like the s in “measure”)
me → /mə/ (mute, very short)
rendors → /ʁɑ̃.dɔʁ/
- final -s is silent
- en → nasal /ɑ̃/ (don’t fully pronounce the n)
rapidement → usually /ʁa.pid.mɑ̃/ in everyday speech (the e in pide often disappears)
après → /a.pʁɛ/ (final -s is silent)
le → /lə/
cauchemar → /koʃ.maʁ/
- au → /o/
- ch → /ʃ/ like “sh”
- final -r is pronounced in standard French

There are no required liaisons in this sentence; each word is pronounced pretty separately.

So the whole sentence is roughly:
/ʒə mə ʁɑ̃.dɔʁ ʁa.pid.mɑ̃ a.pʁɛ lə koʃ.maʁ/.

Is there any difference in meaning if I say Je me rendors vite après le cauchemar instead of Je me rendors rapidement après le cauchemar?

Both mean essentially the same thing: “I go back to sleep quickly after the nightmare.”

Nuance:

rapidement – a bit more neutral or formal, often used in writing.
vite – very common in spoken French, slightly more informal, can feel more direct.

In everyday conversation, Je me rendors vite après le cauchemar would probably sound more natural, but your original sentence is absolutely correct and idiomatic.