Learn Portuguese (Portugal): Já sabemos que a testemunha é inocente. - We already know that the witness is innocent.

The subjunctive is used for doubt, emotion, and uncertainty. The verb saber (to know) expresses certainty and established facts. Because sabemos que (we know that) states a certainty, the following verb stays in the normal indicative mood (é).

If the witness is a man, would we change the phrase to "o testemunha"?

No. Testemunha is a special type of noun that is always grammatically feminine (a testemunha), regardless of whether the actual person is male or female.

Does the adjective "inocente" change if it describes a masculine noun instead?

No, adjectives that end in -e in the singular usually have only one form. They do not change to match masculine or feminine nouns. Therefore, whether it describes a male or female subject, it remains inocente.