I know people who have spoken English for over 10 years, but still give themselves away as non-native speakers by using the incorrect tense.
A common one is that they'll type something like "I didn't understood what he said". I can see their reasoning here: "understood" seems like the correct word to use when talking about a past event. Sadly, I don't know enough about my own native language to explain to them why they should be using the word "understand" instead.
That's actually quite a common rule found in many languages Usually the second or third verbs in a sentence take their infinitive form.
If you want to learn more about English and be able to help people in these cases, the best way is to learn a foreign language. I recommend French or German and get the corresponding "English Grammar for Students of..." book, i.e. for French: https://www.amazon.co.uk/English-Grammar-Students-French-Lea...
These books are excellent. You can read it in a day. You won't be that close to being able to speak French (as you won't know any vocabulary for a start), but you'll suddenly appreciate the grammar of your own language. It will even make you appreciate programming languages more. It's a truly enlightening experience. All the second-language European speakers you meet will know this stuff already.
"Understand" is an an infinite verb. "Understood" and "did" are finite ones. Each clause can have only one finite verb (its head) so "did" and "understood" cannot both occur in one. Put another way, "did" only takes an infinite verb as its argument.
A common one is that they'll type something like "I didn't understood what he said". I can see their reasoning here: "understood" seems like the correct word to use when talking about a past event. Sadly, I don't know enough about my own native language to explain to them why they should be using the word "understand" instead.