Yoda does use the past tense, though not very often:

From The Empire Strikes Back:

This one a long time have I watched. All his life has he looked away... to the future, to the horizon. Never his mind on where he was. Hmm? What he was doing. Hmph. Adventure. Heh! Excitement. Heh! A Jedi craves not these things.

From Return of the Jedi:

No. Unfortunate that you rushed to face him... that incomplete was your training. Not ready for the burden were you.

(Disclaimer: I had to look this stuff up. I assure you, I don't have the Yoda corpus committed to memory.)