ein and eine
What's an easy way to remember when I use eine or ein?
Example of what I mean: Ich habe eine kuh
Why do I use eine in that sentence, and not ein
leran the words with their genders, I suggest to do that with using the defined article.
cow - die Kuh, car - das Auto, guy - der Kerl
Now we have half of what is needed.
The next step is to understand what kasus is used in german. The Gender, the Numerus and the Kasus define how the inflection needs to be.
Ich habe eine Kuh. (eine Kuh = Akkusativ in this example)
Ich gebe einer Kuh etwas zu essen. (einer Kuh = Dativ in this example).
Ein is for masculine and neuter words. Eine is for feminine words. Kuh is a feminine word, so you should use eine, not ein. There are very few rules to know the gender of a word. And unfortunately they work better with abstract things. I recommend you to learn the gender for each word. Instead of learning "Kuh", learn "die Kuh".