Learn English – Second conditional when making plans for the future: “If it rained tomorrow, I would go to the cinema.”

I think the difference between the two types of examples that you've exhibited is the relative placement in time of the action in the "if" clause, and the action in the other clause.

• If it rains in the evening, we won't go for a walk - here, the event of raining occurs BEFORE the decision about whether to go for a walk.
• My teeth will rot if I eat too much sugar - presumably, I'll be eating the sugar BEFORE my teeth rot.
• If it will significantly increase complexity, don't implement this feature - here, the increasing of complexity occurs AFTER the implementation of the feature.
• I will give you money if it will make you happier - here, you becoming happier occurs AFTER I give you the money.

In all the cases where the "if" part happens first chronologically, we use the present tense. In the cases where the "if" part happens second, we use the future tense. However, because sentences of the first type are far more common than sentences in the second type, a good rule for learners to adopt is "don't use the future tense with IF".

You have a mixed conditional, not a first conditional. Your sentence gives advice to another person, and is not a statement of you own personal purpose.

If it rains, I will go tomorrow is a statement of what you would do given the condition of If it rains.

If it rains, I would go tomorrow is advice given to another person as to their best actions given the condition If it rains. In spoken English it would be said as If it rains, I'd go tomorrow.

If it rained, I would go tomorrow is a conditional that suggest that you will perform an action that you do not think is likely to happen.