This topic has always confused me. When I describe someone, do I use adjectives? For example: Дети забавные. Or, would I use "смешно": Дети смешно. As you can see, I am confused about describing people, because I don't know if I should be using adjectives, or predicative. Also, if I were describing someones personality, wouldnt the person need to be in the dative case? I just need a big explanation on describing someones look/personality.
I am not sure if juxtaposing adjectives and predicatives is the correct move here. One is a part of speech and the other is a part of a sentence.
If I am not mistaken, in impersonal sentences the function of predicative may be exercised by adverbs, passive participles and nouns. In personal sentences, which your sentence appears to be, long or short form adjectives may too serve as predicatives. They just have to agree with the subject in number and gender. Hence:
Дети смешны or Дети смешные (not Дети смешно).
Их упрямство смешно (short neuter adjective).
With regard to the example from your comment: ему приятно и уверенно:
This appears to be an impossible mixture of a personal and impersonal sentences. Ему приятно is impersonal but Он уверен must be personal.
As theory goes, non-verbal predicates in impersonal sentences describe a condition or state. In your example, pleasure (being or feeling pleased) is the state the He is in. In the personal fragment of your sentence, however, the non-verbal predicate must describe a quality of the subject. Although one may indeed be put in the state of being assured, I am struggling to think of a Russian word to convey that meaning. Confidence, on the other hand, is a quality of the subject, not the state the subject is put in. Hence Он уверен(ный). So we deal with an adverb and an adjective here. Each of them must agree with its own subject and they cannot agree with each other.
If, however, you chose to describe tranquility instead of confidence, that can be expressed as both a state and a quality:
Ему приятно и спокойно vs Ему приятно, и он спокоен.
Does it make sense?
I thinks you are confusing describing someone's properties and describing their state or feelings (plus some special expressions).
When you describe someone or something as being big, old, short, funny, blue, shabby, you do it the same as in English, using a subject in Nominative and an adjective (which is usually NOT short):
- Его дети умные.
- Эта машина жёлтая.
- Дом очень старый.
- Твои ученики очень смешные = Your pupils are very funny.
When you need a state instead, there is often a typical Dative + predicate construction (with a verb or an predicate adjective). Here, can find adjectives that aren't used in the same meaning when in the full form (or at all):
- Детям смешно = Kids are laughing/ It makes kids laugh (hard to translate exactly , but basically, children "feel that the situation makes them laugh )"
- Машине нужен ремонт = The car needs some repairs.
- Мне холодно = I am cold (or 'feel cold', to be more exact, unless I am a corpse in the morgue)
- Дому пришлось несладко. = The house has had it rough.
- Нам кажется, белые стены лучше смотрятся. = We think (lit. "it seems to us") that the white wall look better.
There are more adjectives and verbs that takes Dative to express something very vaguely classified as "state"/"feeling"/"experience". Also, Dative is used when telling age:
- Ей 30 лет. = She is 30 years old.
A bit more about predicative adjectives in Russian. Actually, sometimes you CAN use a short form adjective but it only works for adjective describing qualities (not classifying something as "wooden" or "American") and must be learnt on a per-word basis:
- Его дети умны → Sounds bookish
- Дом очень стар → a bit old-fashioned
- Моя мама красива. → a bit pompous
- Сопротивление на этом участке бесконечно (Resistance is infinite in this span of a circuit) → just right!
- Мой брат очень силён → Okayish, long form still better
You will also find some adjectives that are only used here, or at least have a distinct meaning here (рад, уверен, похож, мал, велик, должен etc.). It may mean a contextually-appropriate quality, like a piece of cloth being too tight/too big (for a person who is currentyl trying it on). Then it is easy for you because you can memorize a particular adjective having this distinct meaning in short form, and not only changing a shade of meaning or style.
Long story short, with a random adjective it is best you reword your sentence so that the adjective is no longer a predicate. That will actually better than both options in the majority of the general-use cases:
- Это старый дом.
- Это жёлтая машина.
- У него умные дети.
- У него смешные ученики.
- Это прочный, добротный стол.
- В нашем доме стоят новые лифты.
By the way, the example by Avi Gordon shows how important it is to know the stylistic choices on a word-by-word basis when you are dealing with popular adjectives:
Их упрямство смешно.
You cannot use a long adjective here, as it would mean that their stubbornness is "a ridiculous one" (compared to other stubbornnesses, probably?) Full-form adjective typically has this connotation. Namely, you place an object in the class of other objects that have the same quality (objective or as compared to some "standard").
It is really easier for you with adjectives that are only used in one form for a particular meaning. For instance, when you are meaning a straight line or a right angle, they are always "прямая" and "прямой угол" (these are mathematical concepts; there are no "more right angles" or "less straight lines").