An alternate in a debate, contest, or conversation is the other person who will take over if the original choice is unable to attend. And if you have no alternative, you have no other choice.
You see how easy it is to understand the meanings of these words once you realize that they all come from the same source.
And keeping in mind that alter means other.
An alteration is of course a change - a making into something other. When you alter your plans, you make other plans.
An altercation is a verbal dispute. When you have an altercation with someone, you have a violent disagreement, a "fight" with words.
