Learn Italian: Bisogna che tu innaffi la pianta ogni mattina. - It is necessary that you water the plant every morning.

It is triggered by the impersonal expression bisogna che (it is necessary that). Expressions of necessity, desire, or opinion followed by che almost always require the subjunctive in the second part of the sentence.

What is the infinitive of innaffi, and how does its spelling work in the subjunctive?

The infinitive is innaffiare (to water). For regular -are verbs in the present subjunctive, the io, tu, and lui/lei forms all end in -i. Since the root of innaffiare already ends in an i, we don't write a double i; it just becomes innaffi.

Why did they include the pronoun tu? Doesn't Italian usually drop subject pronouns?

Yes, Italian usually drops them. However, because the io, tu, and lui/lei forms are completely identical in the present subjunctive, we often add the pronoun to avoid confusion. Here, tu makes it perfectly clear that you need to water the plant, not me or someone else.

Could I say this without using the subjunctive?