There isn't really a set time when little ends start grasping the meaning behind words. At 8 months old they are exploring their world. If yours if getting into things he shouldn't, doing thihe's he shouldn't, etc. it's best to redirect his attention while offering more than just a "no". If he's getting into a drawer in the kitchen, take him somewhere more appropriate and tell him that drawer isn't for him, and then give him something that it ok to play with. If he is starting to hit or bite, tell him it hurts and isn't nice. If you are holding him while he does it, put him down.babies do understand more than you might think, and usually, a brief explanation works better than a no. They understand the intention behind your explanation, whereas individual words can remain a little abstract for a while. And, the more you talk to your baby, the faster he will develop his own language skills.