I'll try to give a partial answer to your question as I do not know exactly what age or level of English are "grades" 2 and 3.
For articles (a) and (an), I'm sure you can explain that both are used with indefinite singular countable nouns, but I suggest that you avoid teaching your students the rule which determines that: (a) is different from (an), and that (an) always precedes nouns which begin with the 5 vowels (a,e,i,o and u).
You'd better point out that articles (a) and (an) are originally the same, but we add the sound /n/ to make it easier for us to pronounce the next word without stoppage of the air flow from our lungs.
For example, ask them to practise saying:" a apple", " a orange" where they'd have to stop to utter the next word after (a), then add the /n/ and compare.
At this stage, I think that students ought to understand that it is to do with the easiness of saying something other than the above mentioned rule: Students will realize that it's easier to say, " a uniform, a university, an umbrella" than sticking by the rule and say," an uniform, an university".
For the Definite Article, I'd suggest that you fall on L1 and compare.
Sorry for being long-winded, yet, I hope you find this of some help.
- For Teachers