Learn Portuguese (Portugal): Isso é a mesa.

In Portuguese, there are different words for 'that' depending on distance. We use Isso when the object is near the person you are talking to, or just slightly out of your own reach. If the table was far away from both of you, you would use aquilo (that over there).

Since mesa is a feminine word, why don't we use a feminine word for 'that' instead of the neutral Isso?

Great observation! Isso is a gender-neutral pronoun. We use it when pointing to an unidentified thing or a general concept ('That thing is...'). Because you are just pointing at an object to identify what it is, you use the neutral Isso. If you wanted to say 'that table' together as one phrase, you would use the feminine matching form: essa mesa.

Why do we use é here instead of a form of the verb estar?

é comes from the verb ser, which we use to talk about permanent facts and identities. Since we are identifying what the object fundamentally is (a table), we use ser. The other 'to be' verb, estar, is used for temporary states or locations, like if you wanted to say where the table is located right now.

How do I know to use a instead of o before mesa?