Fill the correct present perfect or simple past form of the verb from the options given below:
I ______ an accident today. I don't know the man involved.
A) see
B) saw
C) has seen
D) had seen

 Hint: Present perfect tense is a perfective tense used to express action completed in the present.
Simple past tense is a verb tense that expresses actions or states in the past.

Complete answer :
In the given question, the sentence indicates that the accident occurred in the past. Therefore, we will use the simple past tense.

Let us analyze the options given to us in this question:
1) Option (a), 'see', refers to a verb which is in the simple present tense.
Example- I see a boy in the garde, he is smiling at me.
2) Option (b), ‘saw', refers to a verb which is in the simple past tense.
Example- ‘I saw an accident today. I don't know the man involved.’
3) Option (c), ‘has seen', uses ‘has’ with ‘I’ which is grammatically incorrect, since we only use ‘has’ in a sentence in third person.
Example- She has seen the movie thrice already.
4) Option (d), ‘had seen', refers to a verb in past perfect tense, which does not make sense since the next sentence is in simple past tense (‘don’t know’).
Example- He had seen her doing the dishes in the kitchen when he walked past.

Therefore, Option ‘B is the correct answer.

Note: In such a question, keep in mind that the tens of the sentence that is preceding or following the incomplete sentence determines the form of verb tense which is to be used in the incomplete sentence.