Tip: If the words "direct object pronoun" strike horror and panic into your heart, scroll to the cartoon video in the examples explaining them. They're actually pretty easy to figure out.
Or, just learn by example:
These sentences show how to replace specified persons or things by pronouns (le, la, l', les) to avoid repetition:
Je déteste Marie -> Je la déteste
I hate Marie -> I hate her
Je casse le verre -> Je le casse
I break the glass -> I break it
Tu vois Paul et Léa -> Tu les vois
You see Paul and Léa -> You see them
J'utilise la règle -> Je l'utilise
I'm using the ruler -> I'm using it
Notice that you use the pronouns "le/la/l'/les", which agree in gender and number with the person or thing they replace.
Remember that in French, things have gender.
Note also that you put le, la, l', les before the verb!
N.B.: l' is used for feminine or masculine when in front of a vowel or a silent h.
Compare these cases with: