I already have plans. (すでに[予定](https://eikaiwa.dmm.com/uknow/questions/51759/)が入ってる。)
予定のplansは複数形なので注意してください。
I got stuff to do that day.(その日は[やること](https://eikaiwa.dmm.com/uknow/questions/70669/)がある。)
以上の２つの表現が使えますが、それだけだとちょっと冷たく聞こえてしまうかもしれないので、勉強とか仕事とか、具体的に何の予定があるのかをちゃんと説明した方が断り方としては丁寧だと思います。
＜ボキャブラリー＞
have plans = 予定がある
got stuff to do = やることがある（カジュアルな印象）
I'm sorry but I've already made plans. (Close friend)
Thanks for the invitation but I already have a prior engagement (More formal friend)
Thanks bro' but I'm doing something else. (Relaxed hippy friend!)
Three expressions for three slightly different friends. 'bro' is a popular abbreviation of the word 'brother' these days. It is often used with friends or even strangers whose name you don't know.
To take a rain check is used to tell someone that you cannot accept an invitation now, but would like to do so at a later time:
For example: "Mind if I take a rain check on that drink? I have to work late tonight."
To take a rain check とは、後でなら是非受けたいが、今は招待を受け入れられないと伝える時に使います。
例
"Mind if I take a rain check on that drink? I have to work late tonight."
その飲み回、後で参加することできる？遅くまで今日は仕事があるんだ。
You can say :
Sorry, I already have plans.
Maybe next time, I have plans.
Thank you for the invite ,but I already made plans.
このように言えます。
Thank you for the invite ,but I already made plans.
招待をありがとう、また今度ね。今日は予定があるの。
Thank you for the invite ,but I already made plans.
誘ってくれてありがとう、今日は予定があるんだ。
Unfortunately I have already committed myself to another appointment.
Please let's getting together when its convenient for you.
I really want to spend time with you.
Friendships are an important part of our lives.
Open and honest communication is a sure way of keeping friendships going.
Real friends will understand when you cannot spend time with them.
A) That would have been nice but unfortunately i already have plans.
B) I already have plans,next time ?
A) That would have been nice but unfortunately i already have plans.
*Nice-giving pleasure or satisfaction; pleasant or attractive.
Example-"we had a very nice time"
*Unfortunately-it is unfortunate that.
Example-"unfortunately, we do not have the time to interview every applicant"
*Already-before or by now or the time in question.
Example-"Anna has suffered a great deal already"
B) I already have plans,next time ?
*Next time- Maybe some other time
Next time we meet up i will bring john along
I hope this helps :-)
A) That would have been nice but unfortunately i already have plans.
*Nice-嬉しさや満足を表す。良い、気持ちよい、魅力的な.
例-"we had a very nice time"
（私たちはとてもいい時間を過ごした。）
*Unfortunately-不幸にも
例-"unfortunately, we do not have the time to interview every applicant"
（不幸にも、すべての候補者に面接をする時間はない。）
*Already-既に、その時には既に
例-"Anna has suffered a great deal already"
（アンナはすでにかなりの苦痛を受けていた。）
B) I already have plans,next time ?
*Next time- 次回。
例- Next time we meet up i will bring john along
（次回会う時はジョンを連れてくるよ。）
参考になれば幸いです。
ご質問ありがとうございます。
I already have plans. のように英語で表現することができます。
have plans は「予定がある」というニュアンスの英語表現です。
例：
Thanks for inviting me, but I already have plans.
誘ってくれてありがとう、でももう予定あるんだ。
お役に立ちましたでしょうか？
英語学習頑張ってくださいね！
次のように英語で表現することができます：
I already have plans, sorry.
Unfortunately I already have plans.
「先約がある」は英語で I already have plans と表現することができます。
Unfortunately は「残念ながら」という意味になります。
ぜひ参考にしてください。
また何かありましたらいつでも質問してください。
日本 
イギリス 
南アフリカ 
アメリカ合衆国 
