title_body: Where should I start planning my grammar?
upvoted_answer: It would probably be a good way to start with the features you want to express through grammar. Many features can be expressed in different ways, lexically, through morphology, or through syntax. For example, 'subject' could be done through a particle, an affix, or through the position in the sentence.
  You first come up with the list of features, and then you look at the inventory of forms in the grammar. Do you want a complex morphology? English, for example, marks very few features through morphology (plural for nouns, 3rd person singular for verbs, tenses, …) and a lot through word order or by adding auxiliaries.
  The inventory will define the shape of your language; you can also start with that and then assign features to it. But be clear what kind of features you want to be able to express – not every possible distinction needs to be expressed in language. Do you want to mark the gender of the subject in the verb? Perfectly possible, but probably not very useful, unless your language is for a very gender-segregated society.
  Obviously, the more features you want to express, the more complex your grammar will be. As an extreme example, toki pona pretty much only uses word order and a few particles to distinguish between subject/verb/object, and ignores number or gender markers. In Esperanto most work is done by the morphology, which encodes the role in the sentence, gender of nouns, number, and even a range of semantic relationships (eg &quot;room where you do a particular action&quot;).