Leave out what's already clear
An ellipse (ellipsis) drops parts of a sentence that are clear from the situation or from what was just said. In conversation this is the norm. If you answer everything in full sentences, you sound like a textbook.
One answer, two lengths
Full
Je voudrais un café, s'il vous plaît.
I'd like a coffee, please.
Oui, je viens tout de suite.
Yes, I'm coming right away.
Elliptical
Moi, un café, s'il vous plaît.
A coffee for me, please.
Oui, j'arrive.
Yep, coming.
Short answers
The most common ellipsis is the answer that only gives the new part. Agreeing and disagreeing have fixed short forms with the stressed pronoun. After a negative statement you disagree with si and agree with non plus.
A
J'adore ce quartier.
I love this neighbourhood.
B
Moi aussi.
Me too.
A
Mais je ne supporte pas le bruit le soir.
But I can't stand the noise in the evening.
B
Moi non plus.
Me neither.
C
Moi si, ça ne me dérange pas.
I can, it doesn't bother me.
With verbs of opinion, que oui or que non replaces the whole clause.
Il vient ce soir ? – Je crois que oui.
Is he coming tonight? – I think so.
Ça va coûter cher ? – J'espère que non.
Is it going to be expensive? – I hope not.
Sentences without a verb
Many reactions need no verb at all. They're short but not rude, and completely normal among adults in everyday life. Questions often shrink to Et + the topic.
Pas de souci, je m'en occupe.
No worries, I'll take care of it.
Lui prêter encore de l'argent ? Pas question.
Lend him money again? No way.
Le concert est annulé ? Dommage.
The concert's cancelled? Shame.
Et ton nouveau boulot ?
And how's the new job?
Un café ? – Volontiers.
Coffee? – I'd love one.
