There is no word as difficultiness, yes teacher is right and he has explaing very truly. In both ways u can use this, depends on the situation.

Both. Most tenses can follow "when". When I encounter difficulties... When I encountered difficulties... When I have encountered difficulties... When I had encountered difficulties... It depends what you want to say.

"Difficultness" isn't a word. You want the word "difficulties" or "difficulty" depending on your meaning. I think in your sentences "difficulties" works best. "When I've encountered difficulties..." (=present perfect tense) You have had the difficulties at various times in the indefinite past. And since difficulties can still come up, you can say "Whenever I've encountered difficulties..." "Whenever I've encountered difficulties, I've turned to my friends for help. They have always had good advice for me." "When I encountered difficulties..." (=simple past tense) You had the difficulties in the past but not any more. "I'm so glad I quite my job. My boss was a jerk, and when(ever) I encountered difficulties, no one helped me."