Light here meaning lightweight rather than giving a lot of light, correct?
Right. Лёгкая cannot mean 'giving a lot of light'.
Not sure. I wouldn’t use «све́тлая» for something giving the light. I can’t vouch this is 100% incorrect, but this is not something I would use. I'd say «Эта ла́мпа така́я я́ркая» 'This lamp is so bright'.
That makes sense. I guess in English too we'd normally say bright rather than light.
Could you tell me, how does the meaning of ясный compare to яркий and светлый?
I wouldn’t use «ясный» for the lamp either. «Я́ркий» is something that's immediately noticeable, it catches your eye. Metaphorically you could say «я́кий челове́к» to mean someone who stands out.
«Я́сный» is something that can be easily seen, or allows you to see clearly (я́сная пого́да is the weather when you can see the sky), but it doesn't attract attention. Metaphorically it can mean «understandable». «Я́сно» is the common answer, like «I see», to show that you understand the speaker.
«Све́тлый» is something that has a lot of light, or something that is close to white in colour. Metaphorically it can be used to mean something good, like in «све́тлое бу́дущее» 'bright future'.
Yes, we definitely can. But it sounds more formal than "Хорошо, что лампа такая лёгкая", I can't imagine myself saying "Хорошо, что лампа так легка" in a day-to-day conversation, it just feels out-of-place in informal colloquial speech.
Really? Do you use a lot of long adjectives (as opposed to short adjectives) in day-to-day conversation?
Well, it really depends on the adjective, some short adjectives sound better than others. But yes, I feel I use less short adjectives in normal speech and use more of them in writing. E.g. I could write "Она очень знаменита", but when speaking, I'd usually say "Она очень знаменитая".
Thanks! That's good news for me, because there are various rules (rather than one hard rule) for forming and pronouncing short adjectives, and I find it much easier to use longer adjectives when I'm talking to people in Russian
There are very few short adjectives that are acceptable in speech.
A few predicative adjectives only exist in short form (e.g., рад, должен), there are also adjectives that have a distinctive meaning and use when short (болен, мал, велик, похож, доволен), especially when there are dependent words (Ты так похож на неё!).
Some short adjectives assign "states" (жарко, холодно, хорошо, плохо, тепло, ветренно, дождливо) that are observed in general (weather) or experienced by someone (Мне холодно ~ I am cold, I feel cold).
Apart from that, the only place where you can use a wide range of short adjectives is one-word exclamations like "Good!", "Perfect!", "Awful!" "Marvellous!" and so on. Whichever synonim of "terrific" or "awful" you choose—there are plenty—, you always use a short adjectives used in its neuter form: хорошо, отлично, превосходно, классно, клёво, круто, неплохо, чудесно, великолепно, потрясающе / плохо, ужасно, мерзко, погано
Since some of these, namely, exclamations and feelings, are always in neuter and do not even change gender anyhow (some, like можно and нельзя, don't have gender at all), there are different ways to classify such words in Russian. "Category of state" is often considered a separate class that encompasses these words. This course does not focus too much on theory here because all these words are rather similar from a learner's POV.