There can't be much amiss, 'tis clear, To see the rate you drink your beer. (3-4)
In the first stanza, the guy who's complaining about Terence's sad poetry points out that things can't really be all that bad. If Terence is so miserable, why is he drinking his beer so fast? Well, lots of people drink beer fast when something is "amiss" so maybe this guy's logic isn't all that strong. More importantly, though, this shows that Terence is one of the guys. He's not a standoffish smarty-pants. He might be a poet, but he can also drink a few rounds down at the bar with his buddies.
Oh many a peer of England brews Livelier liquor than the Muse, (19-20)
Here Terence makes a comparison between beer and poetic inspiration. On the one hand, we have some very real booze—the beer brewed by rich English guys ("peers"). On the other hand, we have the metaphorical "liquor" that the Muse "brews" to make poets write beautiful poems. For now, Terence admits that it's the brewers who make more exciting, lively stuff. He's playing a long game though—eventually he'll tell us why the muse's liquor is better.
Ale, man, ale's the stuff to drink For fellows whom it hurts to think: (23-24)
This is a pretty crafty couple of lines. They start out sounding like Terence is playing along with his buddy, telling him that he thinks beer is better than poems ("ale's the stuff to drink"). In the end, though, it turns into a burn. A cold burn. Beer is only really the stuff for you if you're an idiot—if it hurts to think. Ouch.