Learn Russian: Это очень трудная задача.

When это is used as a standalone subject meaning "This is" or "That is," it stays in its neutral form это, regardless of the noun's gender. You only change it to the feminine эта if it attaches directly to the noun to mean "this specific thing" (for example, Эта задача очень трудная - This task is very difficult).

Where is the word for "is" in this sentence?

In the present tense, Russian almost never uses the verb "to be" (is/am/are). Instead of saying "This is a difficult task," you simply say "This — very difficult task."

Since we are learning the future tense, how would I say "This WILL BE a very difficult task"?

Because the present tense verb is "invisible," to talk about the future you just insert the future form of the verb "to be" (быть). For a sentence starting with Это, you use the "it" form: будет. The sentence becomes: Это будет очень трудная задача.

How do we know to use the ending -ая on трудная (difficult)?

The noun задача (task) ends in -а, which tells us it is feminine. Adjectives describing it must also take a feminine ending. The dictionary form of "difficult" is the masculine трудный, and swapping the ending to -ая makes it match.