• The verb possum is a compound of the "pot-" from the adjective which means "able, capable" (you haven't studied it yet) and the verb sum.

 
• The verb possum means "to be able".

 
• The present indicative active is simply the prefix "pot-" attached to the present indicative active of sum.
• Present Indicative Active - Possum
• possum - I am able

potes - you are able
potest - he/she/it is able
possumus - we are able
potestis - you (all) are able
possunt - they are able

* Note that in both the 1st singular and plural and in the 3rd plural, the prefix "pot-" has been changed to "pos-" because the form of "sum" begins with an "s." Romans didn't like the sound "ts" so they softened the "pot" to "pos" [it's a phonetics thing].