grammar - What does 辞書形＋といけない mean?

Usually といけない follows ない形 of 辞書形. For example,

• 寝る前に、歯を磨かないといけない。I have to brush my teeth before sleeping.


• 明日までに宿題を出さないといけない。I have to hand in my homework by tomorrow.

Today, I read a new grammar 辞書形＋といけない. I really don't know what this grammar mean. For example,

• 風邪を引くといけないから、暖かい服を着たほうがいい。


• 子供が寂しがるといけないと思って、おもちゃで遊ばせておきました。


• 子供に言い過ぎるといけないと思うが、すぐ口を出してしまう。

The book also provides other but similar meanings for each sentence above as follows.

• ～引いてはから～ is equal to ～引くといけないから～


• ～寂しがってはと思って～ is equal to ～寂しがるといけないと思って～


• ～言い過ぎてはと思うが～ is equal to ～言い過ぎるといけないと思うが～

Any comments and suggestion are appreciated.

Answer

I think that what is important to see here is that the と here is the conditional-と.

• 歯を磨かないと…if you don't brush your teeth


• 風邪を引くと…if you catch a cold

Next step is to understand the いけない part. You can see it as won't go (well) in the meaning of not good.

• 歯を磨かないといけない…if you don't brush your teeth, that'll be not good. (Hence the meaning of: you must brush your teeth.)



• 風邪を引くといけないから、暖かい服を着たほうがいい。…because if you catch a cold it will be no good, you'd better wear a warm piece of clothing. (Hence, you should wear a warm piece of clothing because you don't want to end up catching a cold.)

---

Here ては is a shortened version of てはだめだ・てはいけない・てはならない. Again, ては indicates conditional.

So 引いては~から　≒　引いてはいけない　≒　引くといけない