Learn Arabic (Modern Standard): ساعد الطبيب ستة عشر مريضا.

Arabic numbers from 13 to 19 have a special rule for gender. The counted noun here is مريضا (patient), which is masculine. For these numbers, the ones digit takes the opposite gender of the noun, so the 'six' part is the feminine ستة. The tens digit takes the same gender as the noun, so the 'ten' part is the masculine عشر.

Why is the word for 'patients' written as the singular مريضا instead of a plural?

In Arabic, when you count things using numbers from 11 to 99, the counted noun must always be singular. It must also take the accusative case, which is why مريضا ends with the double 'a' sound (fathatan) and an extra alif.

Would the ending of ستة عشر change if it was not the object of the sentence?

No! Numbers from 11 to 19 (except for the number 12) are 'frozen' in Arabic. Both parts of the number will always end with a fat-ha (an 'a' sound) regardless of their grammatical role in the sentence. It is always ستةَ عشرَ.

Why does the sentence start with ساعد (helped) instead of الطبيب (the doctor)?

This is a great reminder of standard Arabic word order. In a formal verbal sentence, the verb (ساعد) comes first, followed by the subject doing the action (الطبيب), and finally the object receiving the action (ستة عشر مريضا).