One more on the Semi-Slav -- just an introduction, there is a lot of theory to learn in some lines! 
Chess Tips
| MIDDLEGAME TIPS: |
* Open files are useful only if there is an entry point, and in semi-open positions with many minor pieces an entry for a Rook may be lacking. In these cases you may be able to use a minor piece to make the first invasion, but sometimes the open file is not very useful. |