The Most Difficult Aspects of the English Language
The English language is one of the hardest to learn, due to its creation from a mish mash of languages, and its complex rules. There are different rules for making words plural, past tense, and to indicate who is being talked about. By far, the hardest part about learning English is learning the exceptions to the rules.
Take for example making words plural. In most cases, it's just fine to add "s" or "es" to the end of the word, and call it a day. But what happens when you come to words like moose? Goose? How about fish?
The same thing happens with past tense. I cook a meal. I cooked a meal. I play outside. I played outside. I sit at the table. I ... sitted? Words like sat, took, and swam require the person learning English to remember each word individually, because there is really no rule for remembering how to make their present tense counterparts past tense.
Just for fun: Good... gooder? (A neat little rhyme I learned in third grade, and has stuck with me ever since: Good, better, best, never let it rest, till the good gets better, and the better best.)
Words that have their own rules for when you can use them are the trickiest. Chickens lay, people lie. They're lying in their yard over there.
The best trick for teaching this to ESL students or children is to get them to read. Read with people, read by themselves. As they see words being used properly in sentences, they will start to catch on themselves to those tricky exceptions. It's never to early to start reading, either. Children learn best when they're young!