"I knew what you had done" is in no way superior to "I know what you had done". They simply mean different things. The former means you knew at some time in the past what they did at some time in the past before some other time in the past. The latter means you know now what they did at at some time in the past before some other time in the past.
In this sentence, κάνει is the απαρέμφατο form -- perhaps "past participle" is closest as that's the form used in English for form the present perfect and past perfect tenses (but there's a separate passive past participle form in Greek).
The απαρέμφατο is invariable like e.g. "done" in English, and the είχες shows the person and number.
The απαρέμφατο looks like the third person singular of the aorist subjunctive (e.g. the form used after αυτός θα ....).