Learn Portuguese (Portugal): Entendo bem o professor.

In Portuguese, adverbs like bem (well) usually come right after the verb they describe. In English, we often put the adverb at the very end of the sentence, but in Portuguese, saying Entendo o professor bem sounds unnatural.

Where is the word for I in this sentence?

The pronoun eu (I) is left out because the verb ending already shows who is speaking. The verb entender (to understand) becomes entendo for the first person. You only need to add Eu at the beginning if you want to emphasize that you are the one who understands, rather than someone else.

Could I use the verb compreender instead of entender here?

Yes! Both entender and compreender mean to understand, and they are completely interchangeable in this context. Compreendo bem o professor means the exact same thing, though it can sound a tiny bit more formal.

How would this sentence change if the teacher were female?

You would change the masculine article o to the feminine a, and use the feminine noun professora. The sentence would become Entendo bem a professora.