Parts of the following sentence have been given as options. Select the option that contains an error. While I was walking to the store, my friend calls me and ask for a favour.

Correct — A. The opening clause fixes the time as past: While I was walking. The main clause then jumps to the present with calls, and nothing in the sentence licenses that move.

Option (a) carries a second fault as well. The subject my friend is singular, and two verbs joined by 'and' share it, so the pair must read calls ... and asks — not 'and ask'.

Repaired, the sentence runs: 'While I was walking to the store, my friend called me and asked for a favour.'

Why the others are wrong

• (b)
  for a favour — 'Ask for a favour' is the standard collocation. The verb ask takes 'for' before the thing requested, so this segment is correct as printed and cannot be the fault.
• (c)
  to the store — 'Walk to' is the right preposition for a destination. 'To the store' names where the walking was headed, which is ordinary English and carries no error.
• (d)
  While I was walking — The past continuous is exactly what 'while' wants. 'While I was walking' sets an action in progress as the background to a later event, which is the standard use of this tense.

In this wrapper the four options are the segments themselves, and the fault is almost always tense or agreement. Here it is both, inside one segment.

Sequence of tenses: once a sentence anchors itself in the past, later verbs stay in the past unless something in the sentence moves the time forward.

Subject-verb agreement: two verbs joined by 'and' answer to one subject and must both match it. If 'calls' suits 'my friend', then 'ask' cannot — the pair has to be 'calls and asks'.

Key facts

• A singular subject takes a singular verb, so 'my friend' pairs with 'calls' and with 'asks'.
• Two verbs joined by 'and' share one subject and must agree with it in the same way.
• 'While' with the past continuous marks an action in progress that another past event interrupts.

Common traps

• Picking 'While I was walking' because the tense looks elaborate, when the past continuous is exactly right after 'while'.
• Spotting the tense jump, mentally fixing 'calls', and never noticing that 'ask' is unagreed.
• Hunting for a preposition fault in 'to the store' or 'for a favour' when both collocations are standard.

No directly related past PYQ was found.