How do I know when to use lay and when to use lie, and what are the different forms of each verb? I'm always getting them confused.
The verb lay is transitive.
The verb lie is intransitive.
The confusion comes because the past tense of lie is lay:
Few native speakers get this right. Most people would say, "He laid on the table for two hours." Bill Clinton constantly made this mistake in speeches.
Historically, "lay" is a causitive verb formed from "lie", by a process which is now obsolete in English, but has left some other examples: "rise/raise" and "fall/fell".
In the case of "lie", probably because both words are common and the past of "lie" happens to be the same as the present of "lay", they have become generally confused, and for many people they are no longer distinct.
I don't know why the same has not happened to "fall" and "fell", but it may be simply because the verb "fell" is not very common, being used only of trees and enemies.
The notion that most native speakers are making a mistake about how to use language is ludicrous in the extreme. To paraphrase the linguist Dwight Bolinger, the only possible way that language can be made and defined is by how native speakers, in the broadest sense, use it.
To suggest that the majority are making a mistake completely rules out the idea that languages can change. As we all know, languages do change, so that makes the other notion fatuous.
In this case, it isn't an issue of language change. It's simply another bad prescription; actually, using 'bad' is redundant as a prescription is always bad.