What is the error and how to explain the proper form ?

Question

A student says, “My dad did not bought me a dog last year.” You say, You should say, ‘My dad did not buy me a dog last year.’” The student says, “Why? You told me to use the past tense for things that happened before? Last year happened before, right?”  

What is the error and how to explain the proper form?

Answer 1

The error i this sentence is grammatical. "Did not bought" is grammatically incorrect. "Bought" is the past tense of the verb "buy". If you write a verb that is already in the past tense beside another verb (did) written in the past tense, you get a double negative (in this case, a double past-tense subjunctive) which does not work grammatically.

The proper form is "My dad did not buy me a dog last year." Since "Did" is written in the past tense before "buy" which is written in the present tense, the sentence itself then becomes correctly written in the past tense altogether.