What are the most common grammatical errors made by non-native English speakers?

The most common errors vary depending on the native language of the person learning English.

For instance if the learner is a native-German speaker the most common error tends to be using "It gives" in place of "There is/are" constructions. This is because the equivalent phrase in German is "Es gibt".

For Chinese, Russian, and Polish learners the most common problem tends involve whether and when to use articles (a, an, and the) because their languages don't have any.

For Spanish and Portuguese learners using the simple aspect auxiliary verb 'do' tends to be problematic especially in forming questions (do you like...?) and negative answers (I do not like...) because those languages do not use such a construction.

Actually overall, the issues with word order and auxiliary verbs (especially do) tend to be the most common universal problems. This is mainly due to problems with the way grammar is taught in the early parts of an ESL course. Also, most native speakers and ESL teachers don't understand the rules well enough to effectively teach them either.

The most common errors tend to be those involving mother tongue influence. In other words, a student will use either the grammar or the vocabulary of their native language in speaking English which will lead to mistakes.

Take a common structure in many languages which is noun + adjective. In English it is the other way round, but speakers from France (and many other countries) may say, "Have you got a pencil blue?"

I used to have a job tutoring ESL and also spent a summer teaching English in Korea. I found one of the most common difficulties was with pronouns--confusing gendered pronouns, or singular/plural pronouns.

Hi, I teach English in India. Some of the most common errors involve:

Subject-verb agreement: He know my brother.

Incorrect use of articles: I want to visit the Japan.

Word order: I don't know what is he doing.

Using simple-past instead of present-perfect: I did my MBA (in response to "what are your qualifications?")

Using present-perfect instead of simple-past: I have done my MBA in 2004.

Overuse of present continuous: I am going late to the office. Then the boss is scolding me.

Errors with uncountable nouns: I have many softwares and equipments.

Please visit the link below for some specific errors.

I teach ESL to non-English speakers of lots of nationalities and also overseas.

I agree with Tamarack and add that many students have difficulty changing word order for questions.

eg - statement = "I have been to China", question = "Have you been to China?"