Question

When do we use un and une vs du and de la. in french

Answer 1

Short answer
- Un / une = indefinite articles = "a / an / one" (singular, countable).
- Du / de la / de l' / des = partitive articles = "some / (an unspecified amount of)" (mass/uncountable or an unspecified portion).

More detail and examples

1) Un / une
- Use for one whole, countable item.
- J'ai acheté un livre. (I bought a book.)
- Elle a une voiture. (She has a car.)

2) Des
- Plural of un/une = some (plural countable).
- Ils ont des enfants. (They have children / some children.)

3) Du / de la / de l' (partitive)
- Use to talk about an unspecified quantity or part of something (often food, liquids, materials, abstract mass nouns).
- Je veux du pain. (I want some bread.)
- Elle boit de la soupe. (She is drinking some soup.)
- On a de l'eau. (We have some water.)
- du = contraction of de + le (also used as "of the" or "from the"):
- C'est le livre du professeur. (It’s the teacher’s book / the book of the teacher.)

4) After negation
- Indefinite and partitive articles usually become de (or d') after a negative:
- J'ai des pommes → Je n'ai pas de pommes. (I don’t have any apples.)
- Il y a du pain → Il n'y a pas de pain. (There isn’t any bread.)
- Exception: after être, the article can remain:
- Ce n'est pas un problème. (That’s not a problem.)

5) After expressions of quantity
- Use de (not du/de la) after expressions like beaucoup de, peu de, un kilo de, une tasse de:
- Beaucoup de gens, un verre d'eau, peu de temps.

6) Definite article contrast (le / la / les)
- Use le/la/les when speaking about things in general or something specific:
- J'aime le chocolat. (I like chocolate in general.)
- Donne-moi le livre. (Give me the book.)

Quick summary rule of thumb
- If you mean “one/a single” use un / une.
- If you mean “some / an unspecified amount” (especially with mass nouns like bread, water, sugar) use du / de la / de l'.
- In negatives and after quantity words, use de/d'.

If you want, tell me a few French sentences you have trouble with and I’ll mark which article to use and why.