I am going to refer to IRC once again and try to give a good general answer:

( bornfor) I tried asking in here a while back what the grammar-type thing I have
up on my site is lacking, and got little help :p

I am not quite sure how to answer this easily. It does depend on what you want to do with your language. A naming language needs very little in terms of grammar. A real language which can be used in the real world needs more. So allow me just to write a set of questions which should be answered by the conlanger. Sometimes, the answer is that a feature is lacking. Take the sentence “She has eaten the cookie” and “He ate the biscuit”. Rejistanian translates both of them exactly alike (mi’la’ovik isisuvara’het). English obviously does not. OTOH, the sentence “mi’la’meshi’ovik isisuvara’het” and “mi’la’mesit’ovik isisuvara’het” mean something different even though their difference canot be expressed easily in English (the first indicates that s/he possibly ate the cookie/biscuit the second one indicates the same but also that the speaker cannot estimate how likely it is).

  1. What is the word order? Can it change?
  2. What is the morphosyntactic alignment?
  3. How do you say simple sentences?
  4. How do you decline nouns? (and would you ever decline ‘beer’?😉 )
  5. Make it your conlang: Add pronouns and possession.
  6. How do adjectives work? How do adverbs work easily?
  7. It would be better and less cumbersome if your language had comparisons.
  8. How does negation work?
  9. How did I use tenses? How can I start to be using aspects?
  10. What about a trip to the world of articles?
  11. This conlang has this demonstrative and maybe that one as well, so document that (thanks, xvedejas for suggesting this)
  12. How do questions work? Do yes/no questions work? How can you ask for something more complex? Which question words exist?
  13. Relative clauses and subclauses, which both lurk in other sentences, need to be described
  14. What if you mentioned the irrealis and all the other moods?
  15. Make your conlang count (ie: add numbers) and be the first to use ordinal numbers in your conlang
  16. Regarding all the things you inserted to purposefully deviate from your European natlang. I’ll just assume you’d document that anyways…

EDIT: BTW: If you know what you are doing, this list cannot help much. This is just something, which I think someone reading about a language would ask about first. Just going by this list makes the language very euroclone-ish, which Twey very correctly criticized. On the question whether the lack of certain features should be documented, Twey and me disagreed: Twey considers this unneccessary since documented or not, the speakers have to find other ways to express their thoughts.

EDIT2: Of course, if you want anything more detailled as a structure, the one of CALS exists, but might for a newbie appear too scary.