Life In 19x19

Hi everyone, this is my first time posting in this section. I have been playing alot against the crazystone app on my iphone. It totally destroy me at level 10,lol. Sometimes I can beat it at level 9 and level 8. I would like to get more consistent at beating it at level 8/9. This is one of the games I play on level 8. Appreciate all comments

I also have a question about my black 81 at F5. Is that a bad move after I have the 2 space base and I try to expand upwards. whenever I play at F5, my opponent seems to always play at G4 and I feel like I get either cut off or be force into a bad shape.
Thanks for looking!

Some comments.

Bill, thank you so much for the helpful comments. I really learn alot from them.

Seems like I need to focus on being more aggressive in keeping enemies group separated. It really makes a world of difference in the variations you showed me. Allows me to go from defensive to being on the offensive and keeping me one step ahead.