German personal pronouns change form depending on their case. The case is determined by the verb: some verbs take a direct object (accusative), others take an indirect/dative object (dative).

A few things to notice:

• wir and ihr use the exact same form for dative and accusative (uns, euch) — no distinction to worry about there.
• es doesn't stay es in the dative — it becomes ihm, the same as the masculine form.
• sie (she) and sie (they) look identical in the nominative and accusative, but differ in the dative: ihr (her) vs. ihnen (them).
• The formal Sie/Ihnen is always capitalized, in any position in the sentence.

Most verbs (sehen, kennen, lieben, fragen, besuchen, verstehen) take the accusative. A smaller group of common verbs takes the dative instead and simply has to be memorized: helfen, danken, glauben, gefallen, folgen, gehören, passen, gratulieren.