Depuis or il y a??

Question

How do you know whether to use "depuis" or "il y a" to express a period of time. These lessons have had "J'habite à Paris depuis cinq ans" and also "J'ai mangé il y a un heure"

Accepted Answer

Cathy, 
they have different meanings and English translations. 
See attached lessons on depuis, and compare to the lesson you have linked on ‘il y a’.
 Depuis - since/for  time/period
il y a - duration/ period of time ago  
https://progress.lawlessfrench.com/revision/grammar/how-you-use-depuis-since-for-with-le-present-and-not-le-passe-compose-prepositions-of-time
 https://progress.lawlessfrench.com/revision/grammar/when-to-use-either-le-passe-compose-or-le-present-in-negative-sentences-with-depuis