申し訳ありませんが明日は先約があります。

I'm sorry, but I have a previous appointment for tomorrow.

申(もう)し訳(わけ)ありませんが明日(あした)は先約(せんやく)があります。

Mōshiwake arimasen ga ashita wa sen'yaku ga arimasu.

申し訳 (もうしわけ)

apology; excuse

有る (ある)

to be (usu. of inanimate objects); to exist; to live; to have; to be located; to be equipped with; to happen; to come about

が (が)

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

明日 (あした、あす、みょうにち)

tomorrow; near future

は (は)

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

先約 (せんやく)

previous engagement; prior contract

Readings: シン、もう.す、もう.し-、さる

Meanings: have the honor to, sign of the monkey, 3-5PM, ninth sign of Chinese zodiac

Readings: ヤク、わけ

Meanings: translate, reason, circumstance, case

Readings: メイ、ミョウ、ミン、あ.かり、あか.るい、あか.るむ、あか.らむ、あき.らか、あ.ける、 -あ.け、あ.く、あ.くる、あ.かす

Meanings: bright, light

Readings: ニチ、ジツ、ひ、 -び、 -か

Meanings: day, sun, Japan, counter for days

Readings: セン、さき、ま.ず

Meanings: before, ahead, previous, future, precedence

Readings: ヤク、つづ.まる、つづ.める、つづま.やか

Meanings: promise, approximately, shrink