The problems you are having with the infinitive, active and passive participle are quite simple. In morphology you learn that all the derivatives derive from the radicals of the past tense.
EXAMPLES:
1- If the verb has 3 letter root the infinitive is derived from hearing, which means you will have to build a good vocabulary before trying to form it properly. As for the participles they are easily formed on the patterns: فاعل for the active participle, and مفعول for the passive, without any changes if the verb root is strong.
2- If the verb is 3+ letter root the participles are formed in two steps:
A- Replacing the initial of the present tense with مـُ -notice the Dammah-
B- Vowelizing the second to last letter with Kasra for the active participle, and with Fat'ha in the passive.
That concludes the answer to your question. If you want to learn more about this subject or feel free to contact me lessons.
Ustadh Ehab