Coincidence with "zhenghao"
正好 (zhènghǎo) can be used as an adjective or adverb to help express a coincidence, and it could be related to size, volume, quantity, degree, time, etc. This is similar to how we say in English “Oh, this shirt I grabbed was just the right size." or something like that.
Using 正好 with a Noun
When used with a noun, use 正好 right after it.
Noun + 正好（as an Adj.）
- 这 件 衣服 的 大小 正好。The size of these clothes is just right.
- 两 个 人 正好，一 个 说，一 个 演。Two people is perfect. One to speak and one to act.
- 水温 正好，不 冷 也 不 热。The temperature of the water is perfect, not too cold, not too hot.
- 这 个 楼层 正好，不 高 也 不 低。The floor is perfect, not too high, not too low.
- 两 个 房间 正好 ， 我 一间，我爸妈 一间。Two rooms are perfect. One for me and one for my parents.
Using 正好 with a Verb
With just the Verb
When used with a verb, the 正好 goes right before it.
正好 + Verb (as an Adv.)
- 我 正好 要 出去，你 要 我 帮 你 带 什么 东西 吗？I'm just about to leave. What do you want me to bring for you?
- 我现在 正好 没 事 做，可以 帮你 照顾 一下 宝宝。I just happened to have nothing to do now so I can help you with the baby.
- 我 出 门 的 时候正好 下雨了。I was going out the door just as it started to rain.
- 你 来 了！我 正好 要 给 你 打 电话 。You are here! I was just going to call you.
- 他 正好 是 学 这 个 专业 的，也许 可以 帮 你。His major is just this. Maybe he could help you.
With just 得 and a Verb
You can also use 正好 along with 得, as shown in the following examples. This is usually a clause of its own.
Verb + 得 + 正好 (as an Adv.)
- 你 来 得 正好，不 早 也 不 晚。You arrived at just the right time, not too early and not too late.
- 米饭 煮 得 正好，不 硬 也 不 软。The rice was made perfectly, not too hard and not too soft.
- 盐 放 得 正好，不 咸 也 不 淡。The amount of salt you put was just right, not too salty and not too bland.
- 钱 给 得 正好，不 多 也 不 少。The money you gave was just right, not too much and not too little.
- 这 个 地方 你 选 得 正好，离 我们 都 很 近。The place you chose was perfect. It was close to both of us.
Sources and further reading