When "en" or "et" is placed before the noun it acts as the indefinite article, "a" or "an" but when it is used as a suffix, (for example "Hunden") it changes its meaning to "The"

Choose the lesson on definites and / or indefinites and read the Tips section which will explain.

It depends on the grammatical gender of the noun.

Yes, You should only say what you have learned of that word in that language or hover over the word and see the options.

Yes...you can read something without studying it.

The neuter gender doesn't necessarily follow how animate the real world objects are, it is just a grammatical gender. Saying most nouns referring to animate things are usually common gender is more a rule of thumb than a fixed law of the language. Many inanimate objects are also common gender.