Grammar JLPT Level N4 - Lesson 10

Structure of といい:

Verb + と + いい
［い］Adjective + と + いい
［な］Adjective + だ + と + いい
Noun + だ + と + いい

Wishing for others:
(Desired Outcome) Phrase + といい + ね

Wishing for oneself:
(Desired Outcome) Phrase + といい + なぁ
(Desired Outcome) Phrase + といい+んだ⁽¹⁾ + けど⁽²⁾

⁽¹⁾ のだ
⁽²⁾ けれど、けども、けれども、が

Details about といい:

One of the regular uses of the と particle is to express 'if'. However, unlike some other structures (like なら, たら, and ば), と implies that the 'if' result of と is something that will definitely happen, or is a natural consequence of (A) happening. When combined with the い-Adjective いい, it means 'if (A), it would be good'.
This construction may be used with any phrase ending in a word in its standard form.

明日(あした)は学校(がっこう)に行(い)きたくないな~、雪(ゆき)が降(ふ)るといいな。
I don't want to go to school tomorrow. It would be great if it snows.
今夜(こんや)のパーティーは楽(たの)しいといいね！
I hope your party tonight is fun! (Hoping for someone else's sake)
先輩(せんぱい)が言(い)っていることが本当(ほんとう)だといいね！
If what senpai is saying is true, it would be good!
「今夜(こんや)の夕食(ゆうしょく)は魚(さかな)だといいな~」と弟(おとうと)が言(い)っていました。
My little brother was saying 'I hope dinner tonight is fish'.

Because と expresses a 'certain' outcome, the 'good' in this grammar construction often sounds quite strong, leading to といい regularly being translated closer to 'I hope (A)', or 'I wish (A)'.
There are several situations in which this grammar point can be used to express that you are hoping something for yourself, or hoping something for someone else. This is primarily determined by the use of な, or んだけど (hoping for yourself), or ね (hoping for others/mutual benefit).

明日(あした)は晴(は)れるといいな。
I hope it clears up tomorrow. (Hoping for your own benefit)
明日(あした)の試合(しあい)で勝(か)つといいんだけど相手(あいて)が強(つよ)いから自信(じしん)がない。
I hope to win the tournament tomorrow, but since my opponent is formidable, I am not confident. (Hoping for your own benefit)
来週(らいしゅう)は彼氏(かれし)が来(く)るといいね。
I hope your boyfriend comes next week. (Hoping for another's benefit)

といい

Usage Notes for といい:

When this expression finishes with けど, けれども, or が, it can imply that there are some external factors that will make the desired result difficult to achieve.

明日(あした)の朝(あさ)に荷物(にもつ)が届(とど)くといいんだけど。（今夜(こんや)から雪(ゆき)が降(ふ)るから配達(はいたつ)が大変(たいへん)そう）
I hope my package arrives tomorrow morning. (However, it is going to start snowing tonight, so delivery seems like an issue)
宝(たから)くじに当(あ)たるといいんだが。（一枚(いちまい)しか買(か)っていないから当(あ)たらないと思(おも)う）
I hope I win the lottery. (But since I only bought one ticket, I don't think I will)