What is the meaning of the following pair word: if and buts

What's the meaning of this word pair: rain or shine

If + present simple , + will/can/may + base verb

If + past perfect, + would/could/should have + past participle

If + past simple, + would/could/might + base verb

If + future simple , + will/can/may + base verb

If + , would/could/should have + past participle

If + past participle, + will/can/may + base verb

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