How to use but as a conjunction

But is a coordinating conjunction. You use but to join two ideas or statements. The second idea or statement usually contrasts the first one in some way. 

Main idea +but+ Contrast 

• I don't know his address, but my wife does. 
• I invited her to the party, but she politely refused.
• We tried our best, but we couldn't do it. 
• My computer is very old, but it works properly.  

Sorry/Excuse me+but 

You can use but after an expression of apology to introduce a statement or question politely.  

• I'm sorry, but I can't promise anything.  
• Excuse me, but may I say something? 
• Excuse me, but I don't think so.  
• Forgive me for asking, but why did you divorced your husband? 

For Emphasis 

But can be used between repeated words to give more importance to a word or statement.

• Everyone, but everyone knows the truth. 
• Nobody, but nobody couldn't stop him.  

Except 

But is used after words such as 'nobody', 'everything' or 'anything' to mean except. 

• Everyone but Bob has finished eating. 
• No one but me knows what it means. 
• She's done nothing but sit around and watch television. 

At the beginning of a sentence 

In spoken English you can start a  sentence with but to express surprise, shock, annoyance, etc.  

• 'I've decided to look for a new job. 'But why?' 
• But you promised not to tell anyone. 

But for 

You use but for to say what prevented something from happening.  

• But for heavy snow, I would have arrived earlier for the meeting. (If it weren't for the heavy snow, I'd have arrived earlier...) 

Only 

Only can be used as an adverb to mean only. 

• He's but [=only] a little boy! 
• You can see a lot of birds in our garden:  sparrows, robins and blackbirds, to name but a few.

Linking words:

And