Hi there. The more you listen to English the more you will become familiar with the pronunciation of plurals. It is very difficult to learn rules and then try to remember the rule when you are speaking. You need to practice speaking a lot and listening to others speaking. Is the pronunciation important? It depends on the situation. If you are using English in a business situation, making a presentation, talking to managers etc, it would be important to show that your English is 'good'. If you use your English to talk to friends, then it is not so important. The important thing is if they can understand you. If they can, don't worry too much about it. Just get lots of practice as I said before.