1. "My bags are light" would be ok, another way to say the opposite is to just say "My bags are not heavy." 2. Saying "I put much sugar in my coffee" isn't really correct. If you want to say that you put a great quantity of sugar in your coffee, then "I put a lot of sugar in my coffee" would do just fine. If you're trying to say that you put more than you want, then you could say "I put TOO much sugar in my coffee."

1. 'Light' is fine. To make it sound more natural, I would add an adverb, for example, "My bags are really light". 2. In theory, "I always put much sugar in my coffee" IS grammatically correct - but we never say this. Using 'much' in an affirmative statement sounds strange, old-fashioned and stilted to the modern ear. Native speakers always use an alternative, depending on the context: a) Neutral/Informal ----> a lot of, lots of (or more informally, 'loads of' or other alternatives) b) Semi-formal/Formal ----> a great deal of, a large ( or considerable/substantial) amount of 3. 'Traffic lights' are often plural, in the same way that 'scissors' or 'scales' are a single object that is grammatically plural. We tend to make them singular by saying 'a set of traffic lights'. Each 'light' actually usually has three lights, each of a different colour, and if you say 'The lights were red' it means that the red one was illuminated. What's more, at most junctions, there are several sets of lights involved, on different sides of the road and facing in different directions.

1) You can describe something which is not heavy as "light". That is correct. 2) "A lot of" is correct, but "much" is incorrect. The word "much" is used with uncountable nouns, and it would be ok to say "I don't put much sugar in my coffee" or "I put too much sugar in my coffee", but you can't use it on its own in this context. 3) You are correct, in this case, "light" should be singular. But is somebody arrives somewhere late, they might say "the lights were red", meaning that they had to stop for red lights several times during their drive.