While coming across unknown words may be frustrating, why not look them up online or in a dictionary. The extra effort involved helps one learn or retain words. The next time you see that word you might find yourself thinking, "There's that darned word I had to go look up again! Oh yeah, it meant..."
Mangiare (to eat), bere (to drink), and all other verbs have different conjugations depending on who is doing the action (and what tense the sentence is, but right now we're just doing present tense). The conjugations actually follow a pretty regular pattern (except for irregular verbs like essere). Decent table of endings here: http://italian.about.com/library/fare/blverbs01.htm What that's saying is that for verbs that end in -are, to conjugate, you replace the -are with the listed ending (like -iamo for 'we'), and so on.
So mangiare conjugates as: io mangio (I eat); tu mangi (you eat); lui/lei/Lei mangia (he/she/you [formal] eat); noi mangiamo (we eat); voi mangiate (you all eat); loro mangiano (they eat).
Bere is a little harder because with that table you'd think it's bo, bi, be, etc. But that sounds kind of weird, right? Bere is an irregular verb, so we have to change it a bit by including that 'v' (the changes for irregular verbs vary; you just have to get used to how they sound). So it becomes bevo, bevi, beve, etc.
I wish duolingo would give a more structured grammar lesson instead of teaching individual conjugations over a couple lessons. It's so much easier to remember when you know the pattern to work off of.
if you say "loro mangiano pane" without the article, I understand you don't know Italian very well, that you're trying to communicate and that you're at a basic level. I can imagine a person saying those words while pointing every single thing with his finger. The grammar could seem perfect but we don't speak like that, at all. We use the articles in sentences as "io mangio IL pesce, La frutta; io bevo IL tè, L'acqua, La birra". There are cases where we don't use the article and they could sound odd while learning Italian language. One case is when you don't use the pronoun, e.g. Mangio pane. Bevo birra. In this case we use also the partitive: mangio del pane = io mangio del pane = mangio pane, but "io mangio pane" sounds strange to an italian ear, anyway, we usually don't use the pronoun and use often the partitive. Another case is when you use more than one object and you can use the partitive, too. E.g. Mangio pane e cioccolato. Bevo birra e mangio patatine (or Bevo della birra e mangio delle patatine, it is the same, you can use both, with o without the partitive).
I found a good site that helps learn the grammar of this word. Basically what the site says is that depending on the suffix of the word, or rather depending on the word in front of, 'mang,' it will determine the suffix. So, correct me if I'm wrong, I believe it would be fully acceptable to just say, "Mangiano pane," because the suffix refers to the word, "Loro," so the loro is kind of redundant. The site might explain it better. There's a list of the word to suffix phrases. Pronoun - Verb Ending Io - o Tu - i Lui - a Lei - a Noi - iamo Voi - iate Loro - iano
Heres the link if you want a better teacher: http://groovylearnitalian.com/italian-grammar/italian-tutorial-30-our-first-verb-conjugation-with-to-eat/