Using 'can' and 'able' correctly

I just wrote a small paragraph using: can, can't, able and unable. Did I get everything right?

As soon as I arrived in at my destination, I was able to find the man who was supposed to give me the secret pen drive. Can he be trusted? I asked myself. It doesn't matter. I can't know for sure. As I approached him, he blabbered something in - as I believe - Russian. But I was unable to understand anything he said.

not bad for grammar, but an english speaker, or novelist like myself would make other changes. You change tenses from present to past.