My understanding is that the definite article is required for language nouns, except with "parler", in which case it is optional (and most commonly dropped). So Duo should accept "Il parle anglais" or "Il parle l'anglais".
I had the EXACT correct answer and it marked it as wrong. I have been getting this a LOT lately. Sometimes the answer did not have an accent mark or a circonflex, but the remarks used to come back as correct (but remember to pay attention them). Now it just marks the answer as wrong. I do not know how to add those diacritical marks. This is getting REALLY frustrating. I have to quit the lesson as it just keeps going in an endless loop until you get the answer correct. There is no way to get the correct answer in these situations.
With a second person, singular subject ("tu parles").
You need to memorize the conjugations of (at least) the regular -er verbs: je parle, tu parles, il/elle/on parle, nous parlons, vous parlez, ils/elles parlent.
The root is found by dropping the "er" ("parler" -> "parl") and then you add the endings "e" / "es" / "e" / "ons" / "ez" / "ent" based on the subject.
You have to conjugate a verb to match its subject. There is a lot of information about this key skill; try an Internet search for "French verb conjugation". "parler" is a regular "-er" verb (the most common kind), so it follows a standard set of rules. You can find the conjugation in an online dictionary, such as at https://www.collinsdictionary.com/dictionary/french-english/conjugation/parler. In the present tense, it is "parle" if the subject is first person or third person singular, and "parles" for second person singular.
As for gender, you just need to memorize these -- when you learn a new noun, also learn its gender. Once again, I suggest you seek out information on the Internet.
They sound identical. "parle" is used with 1st and 3rd person singular (je and ils/elles), whereas "parles" is used with 2nd person singular (tu). In this sentence, since the subject is "il" ("he"), we know it must be "parle". Some cases may be ambiguous or need further context.