Select the option that can substitute/replace the bracketed word correctly and complete the sentence. I said I (don’t) understand.

Correct — C. The reporting verb is said, which is past, so the verb it reports moves back a step with it. Present don't becomes past didn't.

That is the sequence-of-tenses rule, or backshift: a past reporting verb pulls the reported verb into the past. 'I said I didn't understand' → option (c).

Why the others are wrong

• (a)
  done — Done is a past participle, not a finite verb. 'I said I done understand' has no auxiliary to carry the tense and is not a sentence.
• (b)
  am doing — Am doing is present continuous, which the past said rules out, and it stacks a second verb in front of understand with nothing joining them.
• (d)
  doesn’t — Doesn't disagrees with the subject I, which takes do rather than does, and it is present tense as well, so it fails the backshift said demands.

When a reporting verb is in the past, the verb being reported shifts one tense back. Grammar books call this backshift, or the sequence of tenses.

Present simple moves to past simple (do to did), present continuous to past continuous, and present perfect or past simple to past perfect. 'I say I don't understand' becomes 'I said I didn't understand'.

The bracket here holds an auxiliary, so two conditions must hold at once: it has to be past, and it has to agree with the subject I. Only didn't does both.

Key facts

• A past reporting verb such as said pulls the reported verb one tense back.
• Present simple do or don't backshifts to did or didn't.
• The auxiliary do goes with I, we, you and they, while does goes with he, she and it.

Common traps

• Leaving the reported verb in the present because the speaker still does not understand
• Picking doesn't and reading it as a past form
• Putting a participle or a continuous form where a finite auxiliary is required

No directly related past PYQ was found.