"Morgaŭ ni estos skribantaj al la profesoro."

Translation: Tomorrow we will be writing to the professor.

[zmjb1]

Skribantas? Why plural, and doesnt present tense verbs end in AS.

[mihxal]

Because it's not a verb.

[StephieRice]

It is an adjective about "ni" so must agree with "ni" in plurality

[Markqz]

How is this different than "Morgaŭ ni skribos al la profesoro." ?

[Grant_Ito]

Skribos = will write, which implies the action will be completed.
Estos skribanta = will be writing, which is an ongoing or continuous action. The action might be completed, but not necessarily so. Or perhaps the speaker wants to emphasise that the activity might be lengthy.

[Markqz]

Dankon!

[Grant_Ito]

Nedankinde! :-)