Learn Russian: Кот съел мою рыбу.

This verb is a bit of an exception! It comes from съесть (to eat completely). The base verb is есть (to eat), which has an irregular past tense: ел (masculine), ела (feminine), ело (neuter), and ели (plural). Since кот (cat) is a masculine noun, we use ел and add the prefix съ- to get съел.

Why is there a hard sign (ъ) in the middle of съел?

The hard sign acts as a separator between the prefix с- and the vowel е. It tells you not to blend the consonant and the vowel together. You pronounce it with a tiny break, like "s-yel", keeping the "s" sound hard.

Why does the sentence use мою рыбу instead of the dictionary forms моя рыба?

The fish is the direct object of the sentence because it is the thing being eaten. This means it needs to be in the accusative case (which you may remember from earlier lessons). For feminine words ending in -а like рыба, the ending changes to -у (рыбу). The word for "my" must match this case, so моя changes to мою.

What is the difference between saying Кот ел and Кот съел?