There are these awful things in the English language called gerunds. This crazy verb form with -ing on the end that drives most learners of the English language absolutely crazy. I know, they're horrible, almost as bad as the present perfect tense. They can be mastered though, so don't give up just yet. Here's a link to a page that has a really good explanation for the whole situation and it's fairly straight forward and easy to understand. http://www.englishclub.com/grammar/verbs-gerunds_2.htm
If you happen to be one of my students and want to try your luck on a little quiz try this one. http://www.englisch-hilfen.de/en/exercises/structures/gerund_prepositions.htm
Another good practice for the preposition and gerund form is a fill in the blank exercise which you can find here: http://homepage.bnv-bamberg.de/englischinteraktiv/gerund-preposition.htm There is one for younger students on this site and the instructions are in German. http://www.kico4u.de/english/uebungen/8klasse/gerundnachadjpraep.htm If you are interested in joining a forum for learning English you can try this one, but remember that you need to register to see the reason your answers are wrong, if you get any wrong. http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-82507.php
Here's another explanation page that has more than just gerunds after prepositions on it. http://www.grammar-test.englet.com/preposition-with-gerund and if you are an advanced student and need a little help in understanding gerunds, here's a link to Grammar Girl. She'll help you out, for sure. http://www.quickanddirtytips.com/education/grammar/what-is-a-gerund Okay, so you've read Grammar Girl's explanation and you're ready to give it a go, so try this http://www.seattlecentral.edu/faculty/dloos/Grammar/gerunds_infinitives/gerunds_after_prepositions_exercises1-4.htm and yes, it's an American English site. :) Are you still not sure about the whole situation and just want to practice one more time? I understand, so try this one. http://www.world-english.org/gerundprep.htm
As a teacher, I've found the best way to help students get the hang of this form is by sheer repetition. Give loads of examples and start asking questions using these forms. If the students are new to the game, then answer your own questions and write it out before you ask the questions of them. In the end they will have to make a list and memorize the verbs which require the gerund form, but in the mean time you can make it a little competition to see who can get the most correct. I took the form and made cards out of them and asked students to come up with their own examples, then if they use it correctly they keep the card. The student with the most cards at the end wins the game. If you have a larger class you can make more than one set of cards and put the students in groups to do this. I hope it helps and makes it a little fun.