[Cyclamen]

First, you need to know that japanese verbs can de divided into 2 main groups, "godan" and "ichidan". Every ichidan verb ends in -る. Godan verbs have many endings, including obviously -る.
So when you have a verb you want to put in the past or negative form, you have to know if it's a godan or an ichidan verb first.

Let's take a godan verb ending in 乗る (のる). "to get on" (bike, bus, car, train...)
to make the negative form, drop the る, add ら + ない
乗る -> 乗らない
to make the past for, drop the る and add った
乗る -> 乗った

Let's take an ichidan verb: 見る (みる) "to look"
to make the negative, drop る and add ない
見る -> 見ない
for the pas, drop る and add た
見る -> 見た　　

Be very carfeul when you do this process! Never confuse an ichidan verb with a godan!! For example, the verb 走る　(はしる) "to run" is a Godan verb, so it goes like
走る-> 走らない-> 走った
and NOT
走る->　走ない->　走た