the difference

Hello !!

What is the difference between 'substitute' and 'alternative'. One of my student asked me this evening. I explained to him that all substitues are alternatives but not all alternatives are substitutes. Was I correct?

You were. A substitute is something that can be "swapped in" for another thing. An alternative is another option, but not to the same degree as a substitute.

e.g. Margerine is a substitute FOR butter -- they have similar functions and you can put one in for another.

Going to the movies is an alternative TO reading a book. They're less similar, but you still do one in place of the other.