Keep it simple: Think about this:

Only Arabic-speakers get PhDs. Chris is an Arabic speaker. Therefore Chris gets a PhD.

If gets a PhD, then an Arabic Speaker, written

PhD->Arabic-speaker

Now here's where you don't want to mess up. It says Chris is in that "Arabic-Speaker" portion, or in other words, he's someone in the necessary condition position. But we can't work backwards. We can't say if Chris (the necessary) speaks Arabic then he gets a PhD.

It's easier to see if we use three topics, as it builds on earlier lessons:

A->B->C

We can't say "if C, then B"

To earn a PhD requires resilience. Therefore, if one shows resilience, they earn a PhD.

Makes it clear that this is confusing suff. for necess.: showing resilience doesn't mean they earn a PhD.