Learn Portuguese (Portugal): Qual é o limite de peso da companhia aérea?

In Portuguese, you almost always use Qual é when asking "What is..." for a specific piece of information, like a limit, a name, or a number. You typically only use O que é when asking for a definition, such as asking what a weight limit actually means.

How would I use the vocabulary from this lesson to say "My bag has too much weight"?

You would use demasiado as a quantifier. Because peso (weight) is a masculine singular noun, you would say A minha mala tem demasiado peso. When acting as a quantifier describing a noun rather than an adverb, demasiado must agree in gender and number with that noun.

Why is "weight limit" translated as limite de peso instead of just combining the words?

Unlike English, Portuguese doesn't normally stack nouns together to modify each other. To say "weight limit", you have to link the two nouns with the preposition de (of), literally saying "limit of weight".

Why is "airline" translated as companhia aérea?

Similar to the previous rule, Portuguese uses a noun and an adjective rather than two nouns. companhia means "company", and aérea is the feminine adjective for "aerial" or "related to the air". Since companhia is a feminine noun, the adjective must match its gender.