I think it 'will / is going to' rain today, I saw the forecast on TV last night and it didn't look good.
is going to — we had this information before speaking
Oooh, this food is 50% off! I 'am going to' / 'will' buy it.
will — we decided while speaking.
FUTURE PERFECT OR FUTURE CONTINUOUS — He's working very hard on his English project. By next month, he 'will be completing' / 'will have completed' the project.
will have completed
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