How to understand the "fossilization" phenomenon in second language acquisition and give examples?

The fossilization of second language acquisition is not the fossilization of the target language, but the fossilization of the interlanguage. This is the first point we must know to understand the "fossilization" in second language acquisition.

In the process of learning a second language, learners gradually establish a language system closer to their mother tongue through trial and error. This language system belongs to neither the mother tongue nor the target language, but something in between, that is, "interlanguage".

Although the learner's language, that is, the interlanguage system, which is constructed by second language learners in the process of trial and error of the target language, is also systematic, permeable and dynamic, it is different from the target language because it will produce "fossilization".

The reality is that second language input is often ignored in the actual second language acquisition environment. Foreign teachers are often regarded as a necessary and sufficient guarantee for second language input, but this is not the case, because input needs to follow intelligibility. When the second language learner in the "critical period" says that he doesn't understand, maybe the problem is not with the learner.

Origin of fossilization:

Also in Interlanguage published in 1972, Selinker defined fossilization as "second language learners tend to keep language items, rules and branching systems of non-target languages in their interlanguage, regardless of their age or the number of instruction or courses they receive in the target language". Selinker even equates "rigid structure" with "error".