If you're talking about 入る as a stand-alone intransitive verb, the latter is the older version of the former. Generally you only use the older one in classical Japanese expressions with obsolete grammar and whatnot when you want to sound classical. It's sort of like how you use the verb "have" in a classical way in expressions such as "Well, well, well, what have we here?" If you follow the contemporary English grammar, it should read as "What do we have here?" instead of  "What have we here?" But the modern grammar version doesn't carry exactly the same tone.

The older version isn't that rare, but mostly they appear in fixed phrases, idioms, and the like. Probably it's best to pick up those frequent phrases one by one as you encounter in the wild.

入る (いる) can also be used in conjunction with another verb to form a new verb as in 寝る + 入る = 寝入る (fall asleep). Personally I recommend you treat them as single words rather than combinations of verbs because that's how native speakers intuitively classify them. It wouldn't hurt to know the mechanism behind them though.

You might want to look up in a good monolingual Japanese dictionary if you want to learn grammar/usage beyond this level.

In any case, the correct one for your example sentence is はいる unless it's taken from text in some tricky context.

Last edited by magamo (2012 October 03, 5:00 pm)