Learn Arabic (Modern Standard): حضر المشاركون المؤتمر. - The participants attended the conference.

In Arabic, regular masculine plurals have two possible endings. We use ـون when the word is the subject doing the action. Since the participants are the ones who attended, المشاركون is the subject. If it were the object of a verb or came after a preposition, it would be المشاركين instead.

If "participants" is plural, why is the verb حضر (attended) in the singular form?

In Arabic, when the verb comes before the subject (which is the standard word order), the verb stays in its singular form. Since حضر comes before the plural subject المشاركون, it stays singular. It only needs to match the subject's gender, which is masculine here.

How would this sentence change if the participants were female?

You would use the regular feminine plural ending ـات instead of the masculine one. The word becomes المشاركات. You would also need to add the feminine marker ـت to the verb, making the sentence: حضرت المشاركات المؤتمر.

What is the singular form of المشاركون?

The singular form is مشارك (a male participant). Because it is a regular noun, you simply add the ـون suffix directly to the end of the singular word to make it plural.