The majority of voters is/are // A new line of small, low-priced computers has/have ...
1.
The majority of voters are undecided at this point.
→Can I use “is” instead?

2.
A new line of small, low-priced computers has/have just been launched by the company.
→Which is correct?

3.
There is/are a large number of people on the street.
→Which is correct?

4.
A large number of people is/are …
→Which is correct?

Thanks!

Question

The majority of voters is/are // A new line of small, low-priced computers has/have ...
1.
The majority of voters are undecided at this point.
→Can I use “is” instead?

2.
A new line of small, low-priced computers has/have just been launched by the company.
→Which is correct?

3.
There is/are a large number of people on the street.
→Which is correct?

4.
A large number of people is/are …
→Which is correct?

Thanks!

Answer

1. Yes, and indeed it may be preferred if you are referring to the group as being undecided, rather than its members individually.
2. "Has". "Have" would refer to the computers rather than the line.
3. "Are". The people are on the street, not the number. 
4. Well, this is somewhat like #3, but there are contexts where the verb should be singular, where the focus is on the number rather than the individuals e.g., "A large number of people is expected in the hall, which has a capacity of 1,000 seats".