This is the 17440th most frequent Russian word.
тетради
Notebooks.
Here, 'тетради' is in the plural nominative case, used as the subject of the sentence to describe what is lying on the shelf.
На полке лежат тетради.
Notebooks are lying on the shelf.
In this sentence, 'тетради' is in the plural accusative case, indicating it is the direct object of the action 'bought'.
Я купил три новые тетради.
I bought three new notebooks.
Here, 'тетради' is in the singular dative case, showing that the inclusion pertains to the notebook.