"Neanderthals were human. They buried their dead, used tools, had a complex social structure, employed language, and played musical instruments. Neanderthal anatomy differences are extremely minor and can be for the most part explained as a result of a genetically isolated people that lived a rigorous life in a harsh, cold climate."
A bit more from
http://www.icr.org/pubs/imp/imp-323.htm :
There are a large number of cultural habits that distance Homo sapiens from animals. No other organisms, either living or fossil, made tools to make other complex tools, buried their dead, had controlled use of fire, practiced religious ceremonies, used complex syntax in their spoken grammar, and played musical instruments, yet we know from their fossils that Neanderthal engaged in all.
Deliberate burial of Neanderthal remains is well known from at least 36 sites with a geographical distribution over most of Eurasia (Gowlett, 1994), with at least 20 complete skeletons known (Lewin, 1998). Some graves have stone tools, animal bones, and flowers buried in the ground, along with the Neanderthal remains. At the Uzbekistan Neanderthal site of Teshik-Tash, is a boy's grave surrounded by a ring of mountain goat bones, horns, and levallois tools indicating ritualism of some sort. Burial is known to have occurred in an unnatural posture, which demonstrates that a corpse was not simply dropped into a hole in the earth without preparation (Trinkaus and Shipman, 1992). Burial implies an awareness of the after life and demonstrates the existence of formal ritual. Indication of strong social ties can be inferred from cases where Neanderthal individuals with severe crippling injuries were cared for (i.e., the Shanidar remains).
In 1996, pristine evidence of Neanderthal humanness came to light, when a cave in Slovenia produced a small flute made from the thigh bone of a cave bear. Four precisely aligned holes are punctured on one side of the four-inch-long bone (Folger and Menon, 1997). Thus cultural evidence strongly supports Neanderthal humanness.
Yes, they were human, but they were not Homo Sapiens. Indeed, they were evidently a separate species within the same genus. I know your literal interpretation of the Bible will not allow for such a concept, but until about 50,000 years ago, there were three separate human species on Earth. Up to about 28,000 years ago, there were still two.
Neanderthals were intelligent and cultured beings capable of abstract thought. They were not, however, us.
How can they tell anything about the syntax of their grammar? There are no written records of their language(s).