Well, the word Spirit, pnuema, is declined in this way:
Singular, nominative: pneuma. Example: The Spirit speaks to me.
Singular, genitive: pneumatos. Example: The power of the Spirit came upon me.
Singular, dative: pneumati. Example: I long for the Spirit.
Singular, accusative: pneuma. Example: I receive the the Spirit.
Pural, nominative: pneumata. Example: Spirits of men return to God.
Plural, genitive: pneumaton. Example: The desire of spirits is to serve God.
Plural, dative: pneumasi. Example: God sends His grace to the spirits of men.
Plural, accusative: pneumata. Example: God recieves the spirits of men.
In each of these cases, the English word "spirit" takes the same form, only singular or plural. But in the Greek, each example (or case) each form would be different. (In the case of pneuma, the nominative and the accusative are identical.