When to use de and when to use ag?

MaryKirwan

I know that they both mean "of", like part of. But de is more common than ag apparently. When do I use one, when do I use the other?

dambarude

I presume you're referring to Irish? This would be better off in the Irish forum.

'Ag' doesn't really mean 'of', it indicates possession. It's more like the verb 'to have' in English, but it is used in many different contexts. M.sh. "tá cat ag Áine"

'De' means of, but once again it depends on the context.

Ms.M

Prepositions don't have a fixed meaning in any language; they won't overlap perfectly with another language.
"ag" can mean at. Also to denote possession as dambarude said.
"de" can be used to mean of in some instances.

The vast majority of the time, the tuiseal ginideach is used instead of "of." This is the rule by which (to simplify) when two nouns come together, the second one changes.

e.g the back of the house = ar chúl an tí (an tí is the t.g of an teach)
at the end of the day = ag deireadh an lae (an lá is the t.g of an lá)
in the centre of town = i lár an bhaile (an bhaile is the t.g of an baile)

A lot of grammar books will have a section on verbs and instances that take the different prepositions.
This is probably what you're looking for.

Enkidu

I assume you are referring to the use ag and de in the specific circumstance when they mean of, as in "part of", properly called the partative dative. (as already said ag can mean at, de can mean off and many other things).

In this case de is used for nouns and the singular pronouns:

cuid de na daoine = some of the people
go leor dhe = lots of it

However for plural pronouns (them, us, you(pl.)) you use ag:
go leor acu = many of them

However if you use de, this will always be indefinite:
doras den tigh = a door of the house
teanga den Éirinn = a language of Ireland

To make it definite you must use the genitive:
doras an tí = the door of the house
teanga na hÉireann = the language of Ireland