Learn Spanish (Latin America): Yo leo la carta pero no comprendo. - I read the letter but I don't understand.

Both leer (to read) and comprender (to understand) are regular -er verbs. To say that "I" do the action, you drop the -er ending from the dictionary form and add -o. That gives us leo (I read) and comprendo (I understand).

Why is yo (I) used at the beginning, but missing before comprendo?

In Spanish, words like yo are optional because the verb endings (the -o in leo and comprendo) already tell us exactly who is doing the action. The speaker chose to include yo at the beginning, but didn't need to repeat it in the second half of the sentence because it is still obvious who is talking.

Does carta mean a letter of the alphabet, or a letter you send in the mail?

It means a letter you send to someone! If you are talking about a letter of the alphabet (like A, B, or C), you would use the word letra instead.

Why isn't there a word for "don't" in no comprendo?

In English, we need helper words like "do" or "don't" to make a sentence negative. Spanish is much simpler: you just put no directly in front of the verb to say you don't do it.