If you need to already know what the speaket meant in order to understand them, then there is no point in communication.
Human language has a pretty clear distinction between syntax and sementics. This is how we recognize that "colorless green ideas sleep furiously" are perfectly well formed, if meaningless. In contrast, "I is happy" is meaningful and unambiguous, but grammatically incorrect.
In terms of syntax, English (like most, if not all) languages is literally ambigous.
Consider the sentence structure:
Subject Verb Object Prepositional-Phrase.
This can be either:
(Subject Verb (Object Prepositional Phrase))
Or
(Subject Verb Object ) Prepositional Phrase.
For instance, consider the sentence "I saw a man with binoculars".
In any sense of the word, this example is structually ambigous.
