I have to deal with those fossilised errors as I myself mainly teach adults in their thirties and forties

Obviousy I cannot ignore such errors ... after all most of my students, particularly the upper intermediate and advanced ones express the need �to be corrected

The whole issue �has to do with the difference between learning and acquisition. They have all learnt that the third person singular requires a final "s", �but will make mistakes like "he know / he don�t know", same thing with the past �"she didn �t came", �and so on ....; and just today one of my students gave a nice 15 min presentation but made the same mistake��"more easy than..."�3 times

Knowing the rules �is one thing, putting them into practice and using them accurately is another thing

To me it will always depend on the setting, if it is speaking or writing

In writing, �the more advanced my students are, the stricter I am when such mistakes are made

It �s more difficult when it comes to speaking: I often focus on fluency more than accuracy

Sometimes however, I point out such mistakes, work on them, do some comparisons with their native language �French �, when necessary,�

My conclusion however is that no matter how long you will work and explain those �fossilised errors, nothing will guarantee that they will not be made again .... But it is our job to work on those imperfections