I have difficulty with the plural forms of certain words, especially Latin ones. For example, what is the plural of agenda?
A reliable dictionary, such as the Canadian Oxford Dictionary (2004) or the Gage Canadian Dictionary (2003), has the answers to all your questions.
When you look up the singular form of a word, you will find the plural—if it is irregular or unusual—listed with it. If no plural is listed, you can assume that the word takes an s or es to make it plural.
Agenda, originally the plural form of agendum, is now considered a singular noun and takes a singular verb. For example:
The plural is agendas.