As often confused with like. As is a conjunction. It is followed by a clause. Like is a preposition. It is followed by a noun or a noun equivalent.
- He fought like a lion. (NOT He fought as a lion.)
- He fought as a lion does. (NOT He fought like a lion does.)
As and though
As can be used in the sense of though.
- Though he was sick, he worked diligently.
- Sick as he was, he worked diligently.
- Though he was young, he fought valiantly.
- Young as he was, he fought valiantly.
As if and as though
These expressions are used more or less synonymously.
She spoke as if she knew everything. (= She spoke as though she knew everything.)
It looks as if/ as though it might rain.
It is wrong to use like in this structure, however, it is widely used in colloquial English.
It looks like it might rain. (Incorrect but common in informal English)
So as indicates purpose.
We started early so as to get a good seat.
Such as indicates result.
His actions were such as to offend everyone.
Than is a subordinating conjunction. It is usually followed by an object pronoun or a subject pronoun + verb.
- She is taller than him.
- She is taller than he is.
- She earns more than him.
- She earns more than he does.
A subject pronoun without a following verb is uncommon after than.
She is taller than he. (Less common than ‘She is taller than him’.)