The following words are all feminine and carry su.

• iyam this one
• eSA this one
• sA that one
• asau that one
• kA which one?
• yA the one that...

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Where does the ai of sAgaraM vAnary udaplavataiSA come from?

Rule vRddhireci. That used to be udaplavata plus eSA. If you get an ai again, click the SWAP button and you'll get something like sAgaram eSA vAnary udaplavata, where you can see the a and the e moreclearly.