The first example is correct English in both examples.
1) It is worth a house, These gifts are worth a house. BOTH are correct; the first
sentence refers to a single thing (IT); the second to multiple things (These).
2) It worthes a house. (No such word as 'worthes')
These gifts worth a house. (Sentence fragment- no verb)
3) It is worth nothing. "It" is very important. BOTH are correct: the second sentence
has quote marks(" ") around the word IT. This seems to be a case of refering to
the word "IT". The meaning of the two sentences is an example of OPPOSITES.
4) It worths nothing. (No such word as "worths"): "It" is not important at all. Correct
grammar for 'The word "it" is not an important word.'
Hope this helps.
- For Teachers