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'.
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.
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".
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.
I was struggling with trying to understand why такая is declined as if it's describing a noun when it's clearly an adverb. It made me think that perhaps adverbs describing adjectives are declined, which would be very weird.
Clearly though, the declination is for the same reason that the adjective is using the long form, there is an implied noun at the end of the sentence that both words are modifying. I suppose that the noun would have to be лампа again because to the best of my knowledge Russian does not have a word that translates to "one" in this context.
Seeing the discussion on this page on how short form adjective uses так instead confirms my position.
In some sense, long forms adjectives are always implying an invisible noun when alone:
- Ты тупой / Ты тупая = You are dumb (towards a male/female)
- Вы тупой / Вы тупая - same, only now polite
- Вы тупые = Y'all are dumb
- Ты туп / Ты тупа / Вы тупы - short forms, so we are now dramatic
You can notice a weird thing happening: the short forms instantly lose connection to a real person as soon as you switch to вы. It now does not matter that вы means a single person. But with a long form adjective, it still does.
We also use long form adjectives when you would use adjective + one in English.