According to my Esperanto dictionary on my Android phone, both mean wake up. Which verb is transitive, and which one isn't? How do you use each in a sentence? Thanks!
Veki is the transitive one. Recall that -iĝ- always yields an intransitive verb. You can also look in PIV to check the definition and the transitiveness of a verb.
You use veki when something wakes someone up and you use vekiĝi when someone just wake up by themself.
Here are some example sentences of how to use them:
- La sono de la vekhorloĝo vekis min. = The sound of the alarm woke me up.
- Pro la sono de la vekhorloĝo mi vekiĝis. = I woke up due to the sound of the alarm.
- Veku min je la oka! = Wake me up at eight!
- Ŝi ĉiam vekiĝas je la naŭa. = She always wakes up at nine.
The fact that you can add -iĝ- to the verb shows you that it's transitive. You can't put -iĝ- on an intransitive verb, it just doesn't mean anything.*
So veki is transitive. It means "to wake someone up" (where "someone" can of course include animals or even slumbering supernatural beings of the deep). It's an action that is done to someone, as in @Joffysloffy's examples: La sono de la vekhorloĝo vekis min and Vi veku min je la oka!
If you want to talk about someone just waking up, without saying what woke them up, then you use vekiĝis: Mi vekiĝis je la 9-a ĉi-matene.
*Note to pedants: I am aware of the tiny number of exceptions to this rule, but they're not relevant to this question, and the rule is good enough for 99% of the time.
This question demonstrates one of the limitations of a bilingual dictionary. If the word being used to explain the Esperanto word has more than one meaning, it may be difficult to sort out the difference.
One option you have is to check the words in PIV via vortaro.net
- veki (iun) Interrompi ies dormon
- vekiĝi Rekonsciiĝi el dormo
This shows that veki means to interrupt someone's sleep, and vekiĝi means to become conscius again from sleep.