The grammatical rules are that "il" goes with most masculine singular nouns. "lo" is used with masculine nouns starting with z, s+consonant (not vowel as in succo) gn, pn, ps, x or y.

In languages like Italian and Spanish, where the conjugation of the verb more often than not encodes person and number, it's quite common to just drop the pronoun entirely (hence the term "pro-drop language"). Generally, you only need to say the pronoun explicitly if there's any potential ambiguity (or if you want to specially emphasize it).

It's not required since the conjugation of the verb tells you. Bevo = I drink. Bevi = you drink. The meanings are very specific.

It is. That's the official answer on this page. What did it tell you it was?

The choices are lui beve..., lei beve...., lui beva.... the first one is the correct one. The second one says she drinks, the third is incorrectly conjugated.

Sometimes the program glitches. That should have been reported.

Because "bere" is an -ere verb.

https://i.imgur.com/8atYu1Y.png

It's semi-irregular because the stem is "bev-", but it otherwise follows the -ere template.

Did you flag it and report it there? Mentioning it in here won't lead to any action on Duo's part.