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If the weather improves, we’ll go for a walk. (It is possible or likely that the weather will improve.)

If the weather improved, we could go for a walk. (It is not likely that the weather will improve.)

There are different types of conditions. Some are possible or likely, others are unlikely, and others are impossible:

Grammar Practice - 1st and 2nd Conditionals

grammar-practice-1st-and-2nd-conditionals

Let's do English ESL general grammar practice. fill in 

If the weather improves, we’ll go for a walk. (It is possible or likely that the weather will improve.)…