Learn Dutch: De tas is licht, niet zwaar. - The bag is light, not heavy.

In this sentence, the adjectives come after the noun and the verb (is). You only add the -e ending when the adjective is placed directly before the noun, like in de lichte tas (the light bag). When it comes after the noun, the adjective stays in its basic, dictionary form.

If I wanted to say 'the heavy bag', would zwaar become zwaare tas?

No, it would become de zware tas. Because zwaar has a long vowel sound, we drop one a when adding the -e ending. This keeps the syllable open so the vowel still sounds long. This is a very common spelling rule in Dutch.

Does licht mean light in weight or light in color?

It can mean both, just like in English! In this sentence, we know it means weight because it is contrasted with zwaar (heavy). If we were talking about colors, the opposite of licht would be donker (dark).

Why do we use niet zwaar instead of geen zwaar?

We use niet to say 'not' before an adjective. You only use geen when negating a noun, which translates to 'no' or 'not a' (for example, geen tas means 'no bag').