desum

Desum, deesse, defui is de- plus sum and inherits every irregularity of sum unchanged. It takes a dative of the person or thing that goes short: "mihi deest", it is lacking to me, where English puts the person in the subject and says I lack it. Getting that inversion automatic is most of the work.

Watch the vowels. Deest and deerat are written with two e's but run together as one syllable in verse, and the infinitive deesse is three syllables, not four. The second sense is the one that catches readers out: "amicis deesse" is not to be absent from your friends but to fail them, to leave them in the lurch. Its opposite is supersum, to be left over, to survive, and the pair desum and supersum is a standing Latin way of saying neither too little nor too much.