When to use before or beforehand? [closed]

Both beforehand and before are adverb and both mean the same. (See Reference, below). Then what is the difference between them? Can they be used interchangeably?

Are these sentence correct (and can I swap before with beforehand)?

A: Have you had an avocado before?

B: No, I never had an avocado before.

A: Before I say anything, I must say that you are an honest person.

A: I had missed my bus before I decided to take a taxi.

Reference

• beforehand - Dictionary Definition : Vocabulary.com
• before - Dictionary Definition : Vocabulary.com

Before and beforehand don't mean exactly the same. Beforehand would not be appropriate in any of your example sentences, as is has the sense of doing something in preparation for a later event ('in anticipation', as the definition says).

I missed the bus because I had not checked the timetable beforehand.

When I interviewed [the celebrity] I had read a lot of information about him beforehand.

Your sentences are correct, and you cannot substitute beforehand in any of them.

Beforehand is used in, for example

 I am going to the supermarket, but I am going to get petrol beforehand. 

It usually refers back to something previously mentioned, and means before that. The previous mention may be only implied eg

 You will need to book beforehand

referring to the event, accomodation, etc that needs to be booked.