Student or Learner
Please tell me the difference between the two in meaning a and b:
1a) She caught the bus.
1b) She has caught the bus.
2a) I saw my sister writing his letter.
2b) I saw my sister write his letter.
3a) She’ll be visiting Canada in November.
3b) She’ll visit Canada in November.
1 The second has some connection to now; she might be missing or late, etc.
2 In the first you saw part of her letterwriting, but in the second you were probably there for the whole time.
3 The first can be used to introduce something that is not necessarily part of her plans for Canda; 'She'll be visiting Canada in November, so she will be there on my sister's birthday.'