Learn Russian: Мой муж покупает новый шампунь каждый месяц. - My husband buys a new shampoo every month.

In Russian, to say you do something like 'every day' or 'every month', you do not need a preposition. You just use the word каждый (every) followed by the time word in the bare Accusative case.

If каждый месяц is in the Accusative case, why doesn't its ending change?

The word месяц (month) is a masculine inanimate noun. For masculine things (not people or animals), the Accusative case is identical to the Nominative (dictionary) case. Because of this, the modifier каждый also keeps its dictionary form.

The word шампунь ends in a soft sign (-ь). Doesn't that make it feminine?

Nouns ending in a soft sign can be either masculine or feminine. Шампунь is a classic word that trips learners up: it is actually masculine! That is why it pairs with the masculine adjective новый (new) instead of the feminine новая.

Since новый шампунь is the direct object being bought, why isn't it taking an Accusative case ending like -ую?

It actually is in the Accusative case! Because шампунь is a masculine inanimate object, its Accusative form does not change from the dictionary form. If it were feminine, the adjective would change to новую, but as a masculine object, it stays новый шампунь.

Why do we use the verb (imperfective) instead of its perfective pair?