The sentence that Koichi provides is “It was filled with water” –> 水　を　いっぱい　入れました
I’m stil unsure how to read it though

jkl:
> 水　を　いっぱい　入れました

Based on the fact that お茶を入れる means “make tea,” my guess is that your sentence means something like “…poured a glass of water.”

> Which reading would I use?

One way to find out how words are read is to look them up in a dictionary. For example,

http://dictionary.goo.ne.jp/leaf/je2/4577/m0u/%E5%85%A5%E3%82%8C%E3%82%8B/

Note also definition 16 on that page.

I think it has something to do with learning the difference between when to use the on’yomi and kun’yomi readings. For example if you see a number kanji i.e　七　by itself it would usually be the kun’yomi which is なな but numbers are the exception so only the on’yomi is put in しち so you know not to use なな.

I’m not sure if this is discussed already somewhere but on the radicals page there are 1-stroke and 2-stroke radicals in the 3-stroke radical category such as slide and drop.