American Sign Language (ASL) is a true natural language developed spontaneously by deaf people. ASL bears little resemblance to English. Even at the simple level of word ordering, how you put words in order to make a sentence, English and ASL are very different. ASL has its own grammar and syntax, its own rules for making sentences.
The grammar of ASL is not seen in the signs or words made by the hands, but on subtle signals of facial expression, eye gaze, and mouth movements. Raised eyebrows can indicate a yes/no sentence. They can indicate the topic of a sentence. Furrowed brows can indicate a question such as a where or what question. Subtle lip movements provide adverb information such as some actions was done naturally or normally, hastily or sloppily.
The differences between ASL and English exist even at the level of word meanings. In English, we use the word 'ran' to mean many things: he ran to the store, he ran for office, his nose ran. In ASL, each of those examples would use a different sign.
ASL is as full and as rich of a language as English or any other language. It just happens to be a different language, expressed by the hands, body, and face rather than the mouth and vocal tract.