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According to "ESL Writing" by Mary Ellen Munoz Page, the following sentence:
"I baked an apple pie, but I forgot to put cinnamon in it."
is classified as a compound sentence,
whereas
"Sonia speaks English but does not write it."
is considered to be a simple sentence.
I don't see the difference between these sentences. I would omit the pronoun "I" after "but" in the first sentence, and classify it as a simple sentence, too.
[1] [I baked an apple pie], but [I forgot to put cinnamon in it].
[2] Sonia [speaks English] but [does not write it].
[1] contains a main-clause coordination (bracketed). Each coordinate could stand alone as a sentence, and hence traditional grammar calls this a compound sentence
By contrast, [2] has a lower-level coordination where this time the coordinates are just verb phrases which could not stand alone as sentences, and hence this is called a simple sentence.
A compound sentence has two complete sentences, which means each has a subject and predicate. (The second subject just happens to be the same as the first, but you could also have "I baked an apple pie, but my dog ate it before I had a chance to taste it.")
Your second sentence only has one subject (Sonia).
