дельфины

In a sentence

• Дельфины обитают в морях и океанах.

  Dolphins inhabit seas and oceans.

  Here, 'дельфины' is used in the nominative plural form as the subject of the sentence.

• Мы наблюдали за дельфинами с пляжа.

  We observed the dolphins from the beach.

  Here, 'дельфинами' is used in the instrumental plural form as the object of the preposition 'за'.

• Я часто думаю о дельфинах.

  I often think about dolphins.

  Here, 'дельфинах' is used in the prepositional plural form to specify 'о ком' (about whom).