This used to be more difficult for me to conceptualize until I had someone explain it to me in "real world" terms.
A receiver is going to do one of two basic things: block or run a route. (He may also run a runoff/decoy route but that really doesn't matter here.) you should be able to tell almost immediately what his intentions are at the snap by his body language. If he wants to block, then the defender has every right to ward him off by pretty much all means necessary that isn't a personal foul in nature. One thing in this case that you should be aware of is OPI. If the receiver is blocking and the ball is thrown to a teammate in the vicinity (beyond the LOS) then you have the potential for an offensive foul.
On the other hand, if the receiver wants to run a route then 9-3-4 becomes more relevant. Basically if a receiver wants to run a route then the defender doesn't get the right to do all of the things like "push, pull, ward off, etc...." So how do you define in writing what running a route is? The answer is by inferring from the "same yard line" clause. If the receiver gets to the same yard line then obviously he's not trying to block by getting between the runner and the defender and therefore the DB does not have the contact latitude as described above when it's a running play. In this instance, then there is a reasonably fine line about what the defender can and can't do.
At the start of every passing play, the receiver has a job to do and the defender has a job to do. The receiver's job is to run the prescribed route. The defender of course does not know this route. Furthermore, the defender has an assigned area of responsibility based on the coverage. He also knows where he does and does not have help. For example, a CB knows that if the FS does not have a receiver to cover and is truly free, then he has help to the inside. Conversely, if the safeties all have a man, then the CB's only help is the sideline. Through time (if not already), you will be able to read the coverages and the DB's positioning and body language prior to the snap and figure it where the DB wants his receiver to go. If the WR goes in that direction, you will see less contact as the DB is simply going to let him go that way. It's when the WR wants to go where the DB doesn't want him to go that you'll need to rule on the concepts in 9-3-4. Proper DB techniques usually equal solid coverage and no fouls. Sloppy technique and good WR technique equals more potential for a foul. Examples are almost always going to be defensive holding in that the DB will get beat and he'll hook the receiver as he goes by. Be careful about the illegal use of hands and over officiating the "continuous contact". I have yet to see a supervisor show me a solid example of that foul and many have told me to stay away from it. (If there is anyone on the board who has a good video example of an illegal use of hands foul on a receiver please post). Usually what you will see with "continuous contact" is that it will turn into a DPI/cutoff or similar because the ball will be in the air pretty quick. If the ball isn't thrown to your receiver then you don't have a foul, but you do need to have a preventive "talk to" with your DB about too much contact, especially if it would have been a foul had the pass come your way.
Long story short: 10-yard fouls (and PI) are advantage/disadvantage fouls. The best advice I can give you in these situations is to officiate the result, do not officiate just the action. If you see contact that rises to the level of a foul, the very next thing to do is find the ball. If it's away from you then it's probably not a foul. If the ball is to you or the QB wanted to go to your side but had to adjust because of your illegal action then that's a foul.
Personal fouls should always be called regardless of where the ball is.
Hope this helps. I know it's a rather lengthy response to what you were probably hoping was a simple answer, but the higher the level of play, the more we need to apply higher level concepts. What is a long-winded explanation here is actually pretty simple and quick on the field.
