"The ducks do not want to eat."
Translation:הברווזות לא רוצות לאכול.
Elsewhere, Duolingo accepts both duck and drake as the Hebrew equivalent of barvaz. So barvaz should be accepted here. In fact, in English (at least American usage) the word 'drake' is unusual, except for technical precision. If I ask my butcher for a duck, I am not concerned with its gender. NOTE: Disney's character is Donald Duck, not Donald Drake!!! barvaz should be accepted here.
Because infinitives (to + a verb) don't have a plural form. Just verbs. So, for example the infinitive "לאכול" stays like that no matter if it's referring to a man, woman or a group because it literally means "to eat." But the verb "אוכל" (eat/eating) changes to match the thing/person it refers to. Does that make things a little more clear? Basically the difference is simply that "לאכול" is an infinitive and "אוכל" is a verb. Infinitives don't change and verbs do (as far as masculine/feminine and plurals).
Yes, it does hold true for all verbs. If it is an infinitive it would never be plural, but if it is an actual verb it has four present tense forms. Here are a few examples from five of the seven binyanim.
Present tense verb forms:
אוכל - אוכלת - אוכלים - אוכלות
מדבר - מדברת - מדברים - מדברות
מבין - מבינה - מבינים - מבינות
נכנס - נכנסת - נכנסים - נכנסות
מתחתן - מתחתנת - מתחתנים - מתחתנות
I think is a general rule. It functions in Indo-European languages I know - English, French, Spanish, Occitan, Latin, German, Italian, Russian as well as Hebrew. The infinitive is invariable. I'm curious, now. Does anyone know of a language where the infinitive is NOT invariable?
Well, your question is somewhat wrongly put, because not every language has an infinitive verb form.
But anyway, according to the Wikipedia article on infinitive, Portuguese, Galician, and Sardinian do have variable infinitives. On the other hand, Greek, Bulgarian, Macedonian, Arabic don't have infinitive.
This is the article : https://en.m.wikipedia.org/wiki/Infinitive. It's not extensive, but an interesting read.
The inflections of "אין" can be used for negation instead of "לא" in present tense. אינם is an inflection of the word "אין" for third person plural masculine (הברווזים). Generally you'd use it to sound more formal. There's an inflection table in the "Formal" lesson, I'm not sure if you've reached there yet but there's a table in the Wiktionary entry as well - https://en.wiktionary.org/wiki/%D7%90%D7%99%D7%9F.