I has to be with practice, It is very easy for Arabic native speakers to give you various examples for common cases, but there are some irregular cases, no other way but practice.

It depends on the verb , but usually you can conjugate most of verbs to the present by adding ي in the front of the past tense , Ex: خرج becomes يخرج (to go out) دخل becomes يدخل (to come in) It's for the masculine in case you want to refer to a feminine you should add ت instead of ي So it becomes تخرج ‘ تدخل If you have any other questions don't hesitate to ask. Thanks!