Parseltongue's complexity comes from having to constantly read between the lines to decipher meaning from a sentence. This thought process is what makes a snake seem so shady, because they will try to cater their speech to their listener. This may make a parselmouth seem shady, manipulative, or crafty simply because they will transfer this thought process to any language they speak.
The better the speaker and listener knows each other, the easier it will be to convey meaning, as the speaker will better understand how to word things to get your message across. Likewise, the listener will better know the speaker's colloquialisms.
Furthermore, the sentence structure of Parseltongue is very basic and relies entirely on context. The actual spoken sentence is typically limited to a demonstrative pronoun, a subject, an object, and a verb. In that order.
If an adjective or adverb is used, it comes after the word it is modifying.
Demonstrative pronouns are almost always used in Parseltongue. This is to help clarify what the subject is. More detail will be given in the gender/number section. The only real exceptions are when talking about oneself or the listener. (using 'I' or 'You' as the subject)
To illustrate concepts in Parseltongue grammar, we will sometimes borrow a concept from signed languages (such as American Sign Language or British Sign Language) known as the gloss. This is a way to express - in writing - the grammatical structure of a language which does not have a written form. One creates a gloss for a language by writing the basic vocabulary words of a reference language which does have a written form (such as English) using the grammatical structures and word order of the language we wish to convey.
We can demonstrate both glosses and sentence order by using the English sentence "That black cat runs away quickly." as an example. Translating it into Parseltongue, its gloss becomes "That cat black away run quickly". Here is how it breaks down, using English approximations of Parseltongue grammatical concepts:
Demonstrative Pronoun: That
Subject: Cat. The cat is the one doing the verb.
Adjective: Black. Describing the subject.
Object: Away. Object of the verb ran. That is where the subject is running to.
Verb: Run. What the subject is doing. ("Run" is the basic unconjugated form).
Adverb: Quickly. How the subject is doing the verb.
Be careful. The object could also have a demonstrative pronoun or adjective! If so, the demonstrative pronoun would be placed immediately before the object (and right after the adjective); a demonstrative adjective would be placed immediately after the object (and right before the verb).