PARTICPLE ENDINGS
So how are these participles formed? Let’s take them in order.
The present active declines like the third-declension adjectives constans (constant) or potens (powerful). So take the verb canto, to sing. A dictionary or verb book will give you its four principal parts: canto, cantare, cantavi, cantus.
To make the present participle, you take the stem of the verb (cant-) and add the appropriate endings from the third declension adjectives. So singing in the nominative singular is cantans, and in the nominative plural is cantantes.
The perfect passive participle is formed with the stem of the fourth principle part, to which you add the endings of a regular -us, –a, –um adjective. So cantus-a-um, “having been sung.”
For the future active participle you go back to the present stem and add same adjective endings, this time with the addition of a characteristic –ur-, giving you -urus, -ura, -urum. So cantaturus means “about to sing.”
Try to differentiate the characteristic forms of each participle: present, past, and future. Cantans, cantus and cantaturus