The simple answer is, yes…
… but it’s very rare.
The slightly longer answer is no. Not normally. Bar some form of brain damage or very extreme circumstances. And it depends on what you mean by “forget” and how much you’re forgetting.
The technical term for forgetting—or losing—your native language is
There are people who have completely lost a language.
But it’s rare.
Normally these people left their home country as children and never returned. Their language wasn’t fully developed, and often there’s a psychological element. For example, they were bullied and had a desperate desire to fit into their new country and language.
This won’t happen for most of us, though.
Let’s say you move to a foreign environment. You use your second language all the time and almost never use your native language. What generally what happens is that you find it harder to recall words quickly. You’ll feel like you should know the word… but can’t remember what it is (the “tip of the tongue” effect). Your language will get slow and awkward and take more effort than it should.
You might also end up inserting phrases and mannerisms from the language you use most… even if they’re unnatural. A great example of this that I’ve noticed in Japanese people who have lived in English speaking countries for a while: they start to say, “one of my” in Japanese. For example, “one of my friends” (watashi no tomodachi no hitori) which is very natural in English… but sounds odd in Japanese.
What you’ve got to understand though…
… this isn’t necessarily “losing” your language.
When you know more than one language, all your languages are constantly active in the brain. So to stop you using the wrong language the brain has a suppression mechanism that effectively puts a language to sleep when you’re not using it.
But when you start using it again, it’ll quickly “wake up”.
Incidentally, this is caused by the same process that makes thinking (and speaking) in English hard for you, the ESL Speaker ー something which I cover in detail in Think English, Speak English.
P.S. If you struggle with thinking in English (so you can speak English fluently and naturally) my book, “Think English, Speak English” will show you what to do ー here.
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