Learn German: Das Abendessen war lecker, aber ich habe nicht viel gegessen. - The dinner was delicious, but I didn't eat much.

Irregular past participles usually get a ge- at the beginning and an -en at the end. Because the root verb essen (to eat) already starts with a vowel, adding ge- would create the awkward word geessen. To make it easier to pronounce, German adds an extra g in the middle, giving us gegessen.

Why do we use the simple past word war for the first half, but the Perfekt tense (habe ... gegessen) for the second half?

In spoken German, the Perfekt tense is the standard way to talk about the past. However, a few very common verbs like sein (to be) and haben (to have) are exceptions. People almost always use their simple past forms, like war (was) and hatte (had), because they are shorter and feel more natural to say in conversation.

Why doesn't the word aber change the word order of the second half of the sentence?

The word aber (but) is a 'position zero' conjunction. This means it connects two complete sentences without taking up the first slot in the new sentence. The subject (ich) still takes position 1, and the conjugated helping verb (habe) stays safely in its normal position 2.

Why is nicht placed right in front of here?