Syntax is the study of the principles and rules for the construction of phrases and sentences in natural languages.
It is the part of grammar that represents a speaker’s knowledge of sentences and their structures.

Rules of Syntax
Specifies the correct word order of language: SVO, VSO, VOS, SOV, etc…
1. The President nominated a new Supreme Court justice.
2. *President the new Supreme justice Court a nominated.
Specifies the grammatical relations of a sentence: subject, object, specifier, complement, head, etc…
They provide the information about who is doing what to whom
1.  John loves Mary.
2. Mary loves John.
Specifies constraint that must be adhered to.
(a) *Disa slept the baby.
(b) Disa slept soundly
(a) *The boy found in the house.
(b) The boy found the ball.
It clarifies structural ambiguity.
“For sale: an antique desk suitable for lady with thick legs and large drawers.”
– [a desk] [for lady with thick legs and large drawers]
– [a desk for lady] [with thick legs and large drawers]
It permits speakers to produce and understand limitless number of sentences never heard produced or heard before. This is called the creative aspect of linguistic knowledge.

Grammatical vs. Ungrammatical
Syntactic knowledge qualifies which utterance is grammatical and which one is ungrammatical.
Well-formed or grammatical sentence: a sequence of words that conform to the rules of syntax.
Ill-formed or ungrammatical sentence:  a sequence of words that violate the rules of syntax.
English sample sentences:
Enormous crickets in pink socks danced at the prom.
Colorless green ideas sleep furiously.
A verb crumpled the milk.
*Furiously sleep ideas green colorless.
*Milk the crumpled verb a.
*The boy quickly in the house the ball found.
*Why are you did not eat the food?

Syntactic Categories
Noun (N) – moisture, policy
Verb (V) – melt, remain
Adjective (Adj) – good, intelligent
Preposition (P) – to, near
Adverb (Adv) – slowly, now
Determiner (Det) – the, this
Degree word (Deg) – very, more
Qualifier (Qual) – always, perhaps
Auxiliary (Aux) – will, can
Conjunction (Conj) – and, or

Phrases
Noun Phrase (NP)
NP is formed by a noun alone or a noun plus other words
a. NP –> Det N (the cat)
b. NP –> Det Adj N (those noisy cats)
c. NP –> N (cats)
d. NP –> Adj N (noisy cats)
e. NP –> Pron (she, you)
f. NP –> Det N PP (my friend from Brazil)

Verb Phrase (VP)
VP can consist of a single V or a V plus other words
a. VP –> V (snored)
b. VP –> V + ADV (sleep soundly)
c. VP –> V + NP + PP (walked the dog through the park)
A VP can combine with a subject NP to form a whole S (sentence)
Pat loves music.

Adjective Phrase (ADJP)
ADJPs are often used to modify nouns and thus often appear as elements of NP, for example:
a very expensive watch
Examples of ADJP
smart
very expensive
certain to win
as tall as his father

Adverbial Phrase (ADVP)
ADVPs are often used to modify V and ADJ and ADV
Example:
almost certainly
fiercely
ADVPs also appear as constituents of VP and ADJP
sleep soundly (VP)
fiercely loyal (ADJP)

Prepositional Phrase (PP)
PPs always consist of a preposition plus an NP
Example:
from Morocco
for nothing
A PP can be constituent of wide range of phrases
my friend from Morocco (NP)
angry with Howard and his dog (ADJP)
go to the movies (VP)
separately from the others (ADVP)

Phrase Structure Trees and Rules
A tree diagram with syntactic category information is called a phrase structure tree
The information represented in a PS tree can also be represented by another formal device: phrase structure (PS) rules.