Hi Sofia,
yes you said it..Practice makes perfect...but I understand it must be really frustrating from a Chinese standpoint.
My advice is to, through Italky or for instance Tandem app, find English speakers to talk to from time to time because learning a language is not like learning Math, Science or other matters..."Practice" is paramount! Then watch videos on youtube about this subject and many more, original movies with subtitles (English), listen to radio/songs...in short, just immerse yourself in the language and I'm sure you'll see improvement quickly.
Anyway, for a theoretical help, here a good link that might help you:
Especially this part I extrapolate purposely:
For those instances when students encounter unknown or confusing English plural forms, the following three instructional resources are recommended: (a) the self-questioning strategy, (b) the dictionary, and (c) informational technology. To begin, teachers can directly teach the use of the self-questioning strategy. This is an overarching strategy in that it encompasses all three of the aforementioned resources. Specifically, this strategy provides students with a systematic problem-solving process to use when encountering unknown or confusing plurals. The self-questioning strategy includes the following three steps.
<ul><li>What do I already know about plural forms? Does this word match a rule, or fit into a category that I already know?</li><li>How can I find out more about this plural form? What is the most efficient way to learn about this word? Should I use a dictionary or informational technology?</li><li>What is my strategy for remembering this plural form? How will I remember what I have learned about this word? What is my personal strategy?</li></ul>