Romaji
Watashi wa o-mise ni ikanakutewa ikemasen.
Watashi wa shokuryou wo kai ni, o-mise ni ikanakutewa ikemasen.
Watashi wa ashita no yoru no PARTY no tame no syokuryou wo kaini, o-mise ni ikanakutewa ikemasen.
Dare mo jikan ga nai node, watashi ga ashita no yoru no PARTY no tame no shokuryou wo kai ni o-mise ni ikanakutewa ikemasen.
(Dare mo ashita no yoru no PARTY no tame no syokuryou wo kai ni iku jikan ga nai node, watashi ga ikanakutewa ikemasen.)
English
I have to go to the store.
I have to go to the store and pick up groceries.
I have to go to the store and pick up groceries for tomorrow night’s party.
I have to go to the store and pick up groceries for tomorrow night’s party, because no one else has time to do it.
I could not think of any good phrase of “pick up groceries,” so I translated to “食料 を 買う (to buy foods)”
I will normally say “明日 の 夜 の パーティ の 買出し(kaidashi) を する (picking up groceries for tomorrow night’s part)”