Essentially just memorization. When duolingo introduces a new word, and in most russian dictionaries, there's a stress mark on the stressed vowel. This mark isn't written in practice though.
Люблю is like or love, right? Sometime Duo refuse one or the other translation. "Do you love me?" and "Flies don't like bread." are two of them. English uses love for almost all that is not a formal appreciation. It means from "to like so much" to real love. Is it similar in Russian? How do you say "I love you"?
"Bread" is not a countable noun in English. Therefore we would never say "a bread", "two breads" etc. If you want to get specific about numbers in relation to bread, you have to use countable nouns like "loaf" or "slice" - e.g. "a/one slice of bread, two slices of bread etc." or "a/one loaf of bread, two loaves of bread etc.". But no native English speaker would ever say "Flies do not like a bread". With uncountable nouns, you don't use indefinite articles or numbers. So it's "Flies do not like bread", never "Flies do not like a bread".
I don't know whether Russian has uncountable nouns with its lack of indefinite articles, but I suppose it must do. If you wouldn't use a number with it in Russian, the English equivalent is probably (though maybe not definitely) uncountable as well.
There is another problem with this course. At the beginning when I started the Russian course I suppose to learn conjugation I and conjugation II for singular and plural case. That's how I have learned Italian, Norwegian and English. I have learned the singular and plural conjugation by heart, then I know which one to use.
Bad grammatical agreement. "Doesn't" (contraction of "does not") goes with the third person SINGULAR, not plural. So "A fly doesn't like bread" would be fine, but "Flies doesn't like bread" is grammatically incorrect. You have to say "Flies DON'T (contraction of "do not") like bread".