Learn Hardcore Hausa: Yabanya tana girma, saboda haka noma zai ba da amfanin gona. - The young cereal crops are growing, so farming will yield produce.

Because tana is the feminine singular agreement form used here for an ongoing action. In this sentence, Yabanya is being treated as a feminine singular noun.

This does not have to mean female in a biological sense. In Hausa, many nouns have grammatical gender, so even things that are not people can take masculine or feminine agreement.

Is tana just the Hausa word for is?

Not exactly. tana is doing more than English is.

It shows:

the subject is feminine singular
the action is ongoing / in progress

So tana girma is best understood as it/she is growing, not as a simple one-word translation of is.

What does girma mean here?

Here, girma means growing, becoming bigger, or increasing.

In Hausa, forms like tana are often followed by a verbal noun or verb-like form. So tana girma literally looks more noun-like than English, but the natural meaning is simply is growing.

Why is there no separate word for to be growing?