The verbal auxiliary सकना is similar to the verbal auxiliary “can” in English.
The verb stem of the main verb is followed by an inflected form of the verb सकना.
[verb stem of main verb] [inflected form of सकना]
All inflection for person, number, gender, tense, mood, etc. is in सकना since the main verb always appears as a verb stem.
सकना is only used as an auxiliary in Hindi. It cannot appear without an explicit main verb. In English, for instance, if someone asks “Can you speak Hindi”, one may reply “Yes, I can”. However, in Hindi, the main verb is always required: “हाँ, मैं हिंदी बोल सकता हूँ” (“Yes, I can speak Hindi”).
मैं हिंदी बोल सकता हूँ – “I can speak Hindi”
वह शायद भारत जा सके – “He might be able to go to India”
मैं तबला नहीं बजा सकता – “I can’t play the tabla”
हो सकता है कि हम जीत जाएँ – “It is possible that we will win”, Literally “It can be that we will win”
The subjunctive mood is often used in subordinate clauses like the one in the previous example.
मैं तबला बजा सकता था – “I used to be able to play the tabla”
मैं नहीं जा सका – “I could not go”
मैं भारत जा सकता था – “I used to be able to go to India” OR “I could have gone to India”
Often the sense of सकना is possibility versus ability:
मैं जा सकता हूँ – “I can go” OR “Perhaps I may go”
The negative particle नहीं may either follow or precede the verb stem:
मैं सितार नहीं बजा सकता – “I cannot play the sitar”
मैं सितार बजा नहीं सकता – “I cannot play the sitar”
Note also that often the form of the verb होना is omitted in negative sentences involving the present habitual verb forms, e.g.:
मैं सितार नहीं बजा सकता
is more typical than
मैं सितार नहीं बजा सकता हूँ