Breakdown of Мы кладём документы внутрь папки.
мы
we
документ
document
класть
to put, lay (flat)
папка
folder
Questions & Answers about Мы кладём документы внутрь папки.
Why do we use кладём here instead of лежим?
Кладём is from the transitive verb класть (to put or lay something down). We use it when a subject is actively moving an object into a flat position. Лежать (to lie/be lying) is an intransitive state verb used when the object is already resting somewhere (for example, if the documents were already lying in the folder).
I thought the perfective verb положить meant 'to put'. Why are we using кладём?
Класть and положить form an aspect pair, meaning they are the imperfective and perfective versions of the exact same action. We use the imperfective кладём here because the action is happening right now in the present tense ('We are putting'). The perfective form положим would mean 'We will put' as a completed future action.
Why doesn't документы change its ending, since it is the object being put into the folder?
Документы is indeed in the Accusative case because it is the direct object of the verb кладём. However, because it is an inanimate plural noun, its Accusative form looks exactly the same as its Nominative (dictionary) form.
Why does the word for folder (папка) change to папки at the end of the sentence?
The preposition внутрь (inside) always requires the Genitive case. Since папка is a feminine noun ending in -а, it changes to -и in the Genitive case, giving us внутрь папки.