A few defective verbs, such as sava 'know' and kera 'want' may exceptionally be unmarked in the present (These two verbs have no past form.) Verbs in dependent temporal clauses have optional past and perfective marking to indicate temporal priorness (i.e. past relative to the time of the main verb). [These are the SLP versions of the South Asian “conjunctive past participle”.]
Source: Smith 1974-5: 5180
Source: Smith 1974-5: 3073
Source: Smith 1974-5: 3993