A verb is a doing or action word.
The present tense means that something is happening now.
The children are walking down the road. They walk to school every day.
The past tense is used for something that has already happened.
The children walked to school last week.
We form the past tense of a lot of verbs by adding the suffix ed.
walk + ed = walked
However, there are also a lot of verbs that change completely in the past tense.
We don't say 'I eated my dinner'. The past tense of the verb eat is ate.
I ate all my dinner yesterday.
Unfortunately we just have to learn which verbs form the past tense by adding ed and which ones change!