qatal with modal meaning

[Aleppan]

Is there any rule for deciding when qatal (or suffix conjugation) has a modal meaning?

I read that whenever it has a vav conjunctive before it it has a modal meaning. Is this true?

Like Exodus 7:18

וְהַדָּגָה אֲשֶׁר בַּיְאֹר תָּמוּת וּבָאַשׁ הַיְאֹר וְנִלְאוּ מִצְרַיִם לִשְׁתּוֹת מַיִם מִן הַיְאֹר.
And the fish that are in the Nile will die so that the Nile stinks.

 

[Drink]

qatal with a vav before it essentially flips meanings (usually). So תמות and ובאש are essentially the same tense/aspect/mood in this sentence.

The only way to tell if the meaning is modal is by context.