Learn Russian: Я не хочу жить с таким плохим соседом. - I do not want to live with such a bad neighbor.

The Instrumental case has two main meanings that translate to English as with. When you use an object as a tool (like writing with a pen), you use the bare case with no preposition. But when it means together with a person or thing, like living with a neighbor (с соседом), you must use the preposition с.

How do the adjective and pronoun endings work for таким плохим?

They are describing the masculine noun сосед, which becomes соседом in the Instrumental case. For masculine adjectives and pronouns in this case, the standard endings are -ым or -им. Here, the demonstrative pronoun такой becomes таким and the adjective плохой becomes плохим.

Why do таким and плохим both end in -им instead of the standard -ым?

This is because of the 7-letter spelling rule, which you may remember from earlier lessons. You cannot write the letter ы after к, г, х, ж, ч, ш, or щ. Since the stems of these words end in к and х, we are forced to write и instead of ы.

Why do we use таким instead of так for such or so?