can/could, report writing, 1st,2nd,3rd person, tinkering

[Ju]

What are the differences in meaning of the following sentences?

1) The task could be completed in 2 days.
2) The task can be completed in 2 days.
__________________________________________________ _______

Shall I use past tense always for report writing regardless the fact mentioned ? (eg. a man called Sam who is alive in reality.)

"I told May not to cry because Sam is still alive." ,or,
"I told May not to cry because Sam was still alive."
__________________________________________________ _________

Following are the extracts from a English Grammer book.

1) 'Shall' expresses the future tense when used for the first person,
singular and plural.
2) 'Will' expresses the future tense when used for the second and third
persons, singular and plural.

What does it mean of first ,second and third person ?
__________________________________________________ _________
"As a boy he would always be tinkering with something."

1) What does it mean of 'tinkering' here?
2) Is it OK and what are the difference in meaning if I put a comma
behind "As a boy"?
__________________________________________________ ________

Thanks / ju

[Tdol]

1 The degree of certainty- there might be conditions that need to be met in the first or even no possibility of it being done.

2 You don't have to- you can use the present for facts and things that don't change or when something is known to be the same. Here, I'd use the present if Sam's being alive were important at the time of speaking- if, say, there had been an accident and Sam was in hospital. At a distance from the event, I would probably change it.

3
First person: I/we
Second person: you
Third person: he/she/it
Person - Glossary Definition - UsingEnglish.com

By the way, that's possibly a bit of an old grammar book- the rule doesn't hold true in the US, where 'shall' isn't used much at all, or the UK, where 'shall' is used, but rarely in that way. In a discussion here, I read that the rule is still taught in South Asia but don't know how widely it is used.

4) Playing with, fiddling, adjusting, especially mechanical/electronic things

[Charlie Bernstein]

What are the differences in meaning of the following sentences?

1) The task could be completed in 2 days. Depending on context, it means can, might, or might, under certain conditions.
2) The task can be completed in 2 days. No doubt about it!
__________________________________________________ _______

Shall I use past tense always for report writing regardless the fact mentioned ? (eg. a man called Sam who is alive in reality.)

"I told May not to cry because Sam is still alive." ,or,
"I told May not to cry because Sam was still alive."

Both are good grammar. If Sam is alive when you write this, use is. If you mean he was alive at the time you talked with May, use was.
__________________________________________________ _________

Following are the extracts from a English Grammer book.

1) 'Shall' expresses the future tense when used for the first person,
singular and plural.
2) 'Will' expresses the future tense when used for the second and third
persons, singular and plural.

What does it mean of first ,second and third person ?

You need to know this. It's basic.

First person, present tense: I am, we are.
Second person, present tense: You are, you are.
Third person, present tense: He is, she is, it is, they are.
__________________________________________________ _________
"As a boy he would always be tinkering with something."

1) What does it mean of 'tinkering' here? It means: Open your dictionary!

2) Is it OK and what are the difference in meaning if I put a comma
behind "As a boy"?

A comma there is good. It's not required, because the clause is short and the meaning without a comma is still clear. It means the same thing either way.
__________________________________________________ ________

Thanks / ju

You're welcome!