Onegai shimasu is used when asking someone to do something for you, but unlike kudasai, which needs to be attached to the verb, onegai shimasu can stand by itself, such as in giving an item to buy to the cashier, or to a waiter to get a dirty plate removed, or as an additional exhortation after the initial please, as in "Oshiete kudasai. Onegai shimasu." Yarimasho-ka might also be better for "Let's practice (play) together, shall we?"
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Sempai -> decreasing rank door
Shimoza