My girlfriend is a native speaker of English born in California but she often makes the same mistakes over and over again in spelling…

1. She often confuses there/their/they’re, to/too and your/you’re…
2. She consistently forgets to write the final -s in the third person singular verb conjugation of regular verbs. She also forgets to mark the possessive noun with an apostrophe s. She often writes “The man work in an office” instead of “… works…” and “the woman dog is friendly” instead of “… woman’s…”
3. She consistently forgets to write the final -ed when it comes to the past tense conjugation of verbs or when it comes to the passive voice. She writes “I fix my sink earlier” instead of “… fixed…” and “This show is sponsor by…” when it should be “… sponsored…”

For 2 and 3 she doesn’t make the mistake in spoken English, only written. Will any of you be able to explain her mistakes to her? I tried to explain them but I am not good at that.

Question

My girlfriend is a native speaker of English born in California but she often makes the same mistakes over and over again in spelling…

1. She often confuses there/their/they’re, to/too and your/you’re…
2. She consistently forgets to write the final -s in the third person singular verb conjugation of regular verbs.  She also forgets to mark the possessive noun with an apostrophe s.  She often writes “The man work in an office” instead of “… works…”  and “the woman dog is friendly” instead of “… woman’s…”
3. She consistently forgets to write the final -ed when it comes to the past tense conjugation of verbs or when it comes to the passive voice.  She writes “I fix my sink earlier” instead of “… fixed…” and “This show is sponsor by…” when it should be “… sponsored…”

For 2 and 3 she doesn’t make the mistake in spoken English, only written.  Will any of you be able to explain her mistakes to her? I tried to explain them but I am not good at that.

Answer

There is a type of learning disorder called dysgraphia, where individuals have difficulty writing with correct grammar (and/or a number of other issues with writing specifically such as spelling, spacing, etc. etc.), despite adequate teaching and exposure to the subject area.  It is possible that your girlfriend may have this learning disorder, especially since her spoken English is fine.  It is a poorly understood disorder that is often under diagnosed.

Answer

Maybe go back to college to take ENGL& 101 class again?

Answer

In addition to the above suggestions, if she really wants to improve her grammatical accuracy, I would recommend she study the following:
Essential Grammar in Use
English Grammar in Use
Both by Raymond Murphy. (paperback/app/ebook) 
English grammar is rarely taught in any detail to native speakers in schools, so many people would benefit from a  little help!

Answer

Are her parents native speakers? Being born in a particular location doesn't necessarily mean you will speak the native language to a particular level. (1) isn't uncommon with native speakers. It's annoying, but you see it all over social media; (2) and (3) are more unusual and suggest that her family and community don't speak English as a first language, or that she has a lack of formal education in English language.

Answer

Dear Tsui, 
I would say it is not sufficient to be born in a place to speak a specific language correctly. Maybe was her schooling not adeguate? Or does she come from a bilingual family or her grandparents talk/talked a foreign language, and she might be influenced by another/other language/s/? Some Americans, especially Black-African, might drop 's' in what linguists call 'African American Vernacular English', and this is considered (by experts) a linguistic variation, not broken English.

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