The Times used the headline "Must Try Harder" in a story about the UK's School Minister, Nick Gibb, failing a Sats grammar question aimed at 11 year olds.

A question about subordinating conjunctions was posed to the minister during a BBC Radio Four interview on World At One. He was asked whether the word 'after' in the following sentence, "I went to the cinema after I'd eaten my dinner," was a preposition or a subordinating conjunction.  He answered, "It's a preposition". 

Oops!  

!Note - Teachers in the UK often write "Must try harder" in pupils school reports.  (Mine usually said, "Could do better".) 

For purists: Subordinating conjunctions are used to join a subordinate clause to a main clause.  In other words when one idea in a sentence is dependent upon another. The subordinating conjunction, after, can act as a preposition, if it is followed by an object rather than a dependent clause. But, if the word joins a verb to the sentence, it is acting as a conjunction.

Laika ran after the ball.  (After is a preposition.)

We went out to eat after the cinema. (After is a subordinating conjunction, because we are really saying "after watching the film at the cinema.)

One a final note. I think his failure proves my point - you can read and write to a high level without being a grammarian.