J'ai mangéune pomme hier.
→ Je l'ai mangée sur mon balcon.
Yesterday, I ate an apple. I ate it on my balcony.
Ensuite je me suis lavéles mains.
→ Je meles suis lavées avec du savon.
Then, I washed my hands. I washed them with soap.
Okay... thanks for that information... As for me, I ate a banana, do you need more details?
Non merci, actually. Let's focus on my story, please. Look at the verbs...
Yes, I did notice that the endings were different, even though in both sentences the tense, the verb and the objects are similar...
Well spotted, let's dig in! Take the first sentence and answer this question: what did I eat yesterday?
Oui. And look, une pomme is directly next to the verb, there is no word between. Right,in the second sentence, I am still talking about this same apple but I did not want to repeat the same word, so what did I use?
l' ?I remember we did mention this before.
Good, so in the first sentence 'apple' is directly after the verb and in the second, 'apple' is now before the verb (in the form of l'), do you still follow?
Yes, I see the move of 'apple' in the sentence and let me guess...mangé became mangée because pomme is feminine?
C'est ça ! And look at the second sentence, it also works with the reflexive verbs. More examples for you:
J'ai revu une amie d'enfance. → Je l'ai revue.
I saw a childhood friend again. → I saw her again.
Elle a acheté des glaces. → Elle les a achetées.
She bought ice creams. → She bought them.
Il s'est brossé les cheveux. → Il se les est brossés.
He brushed his hair. → He brushed it.
Remember that the verb only changes in the second sentence if the word is directly after the verb in the first sentence. If there is something between (a preposition), then the verb does not change.