Immediately it should be said that the verb "lay down" does not exist. Even if you write it word to Word it will underline it red, and in the dictionary to find it impossible. Modified forms (past and future) does not exist. To use this verb can only be consoles, for example, to put, to lay, to put , and so on. It is worth noting that these words are just perfect.
The verb to lay should be used without any subscripts. Despite the fact that in olden times there was the word "poklast, now it has dropped out of the speech of turnover and is considered archaic. "Poklady", "perekladi" and the like can not be used. Otherwise, the verb put can be used as you want (in any shape, form, and time): put, gonna put , and so on.
So how to put or lay down? Of course, a put, but nothing prevents you to add a prefix and to use the word "put". However, it is better to use in the case when you want to say that the action is absolutely in the future or has already been committed. The emphasis on duration. For example, don't forget to put the keys back when I get back from town. In other cases it will be appropriate to use the word put.