alternate - alternative
Alternate actions, events, or processes keep happening regularly after each other.
...the alternate contraction and relaxation of muscles.
If something happens on alternate days, it happens on one day, then does not happen on the next day, then happens again on the day after it, and so on. Things can also happen in alternate weeks, months, or years.
We saw each other on alternate Sunday nights.
The two courses are available in alternate years.
You use alternative to describe something that can be used, had, or done instead of something else.
But still people try to find alternative explanations.
There is, however, an alternative approach.
Note that in American English, alternate is sometimes used with this meaning.
How would a clever researcher rule out this alternate explanation?
Alternative can also be a noun. An alternative to something is something else that you can have or do instead.
Food suppliers are working hard to provide organic alternatives to everyday foodstuffs.
A magistrate offered them a Domestic Education course as an alternative to prison.
There is no alternative to permanent storage.
You can also say that someone has two or more alternatives, meaning that they have two or more courses of action to choose from.
If a man is threatened with attack, he has five alternatives: he can fight, flee, hide, summon help, or try to appease his attacker.
Note that it used to be considered incorrect to talk about more than two alternatives.