modal verb
1
BAD : When capital punishment was abolished, people thought that murders will become more numerous.
GOOD : When capital punishment was abolished, people thought that murders would become more numerous.
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A reporting verb in the past tense (e.g. 'thought') is usually followed by would/could (NOT will/can ). Compare: 'I think she will accept any job that comes along.' 'I thought she would accept any job that came along.'
2
BAD : If a developing country will become a developed country, it has to attract foreign investors.
GOOD : If a developing country is to become a developed country, it has to attract foreign investors.
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When you are talking about what must happen in order to make something possible, use if ... am/is/are to do sth (NOT will ): 'If we are to get there by six o'clock, we'll have to get a taxi.'