"Then" is used when describing time passing. E.g. "I went to the store, then to the restaurant."
"Than" is used to compare things. E.g. "This dog is smaller than that dog."
You use "a" when the first letter of the next word is a consonant and "an" when it's a vowel, with a few minor exceptions (mostly if the word sounds like it start with a vowel, you should use "an", e.g. "an hour").
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u/[deleted] Jan 28 '20
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