Do you say major phenomenas or phenomena?

It is phenomena.

Answer 2

Phenomena is the plural of phenomenon. But a lot of people don't realise that. In the same way as criterion is the singular pf criteria. You hear people say things like 'the criteria I want for my loudspeakers is clarity'. There's only one 'criteria' here so it should be 'the criterion I want for my loudspeakers is clarity'.

I've also heard people refer to cactis. The plural of cactus is cacti. So you don't pluralise it again by saying cactis.