Most students of Arabic grammar are afraid of numbers. Even some native speakers have difficulties to write and pronounce them correctly.
Let us check the number ten (10) for example.
In order to find the correct vowel, you have to check the gender of the noun to which it is referring to.
If عشر points to a MASCULINE word, there is a فَتْحة on the letter ش
|I bought 10 pens.|
اشْتَرَيْتُ عشَرةَ أقْلَامٍ
|I bought 13 pens.|
اِشْتَرَيْتُ ثلاثةَ عشَرَ قَلَمًا
If عشر points to a FEMININE word, there is a سُكُون above the letter ش
|I read 10 pages.|
قرأتُ عشْرَ صَفْحاتٍ
|I read 13 pages.|
قرأتُ ثَلاثَ عشْرةَ صَفْحَةً