Than vs Then, a vs the

[B3l1]

I'm not sure when I should use than and when I should use then?
The same situation is with the prepositions a and the, I'm putting a or the only by felling,but I would like to know this once for all times.. if someone can explain it to me, I'll be pleased.

[Beeesneees]

'than' and 'then' have completely different meanings.
http://www.oxfordadvancedlearnersdictionary.com/dictionary/than
http://www.oxfordadvancedlearnersdictionary.com/dictionary/then
http://www.oxfordadvancedlearnersdictionary.com/dictionary/then_2

Use 'a/an' when you are talking about a noun generally.
Use 'the' if you are talking about one particular example of that noun.
There is a tree in our garden. ('tree' in general)
The tree in our garden is an oak tree. (This particular tree)
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Cheers m' dears!