Starting with the illustration that opens the article, I invite you, dear user, to observe both placements, once expressed by:
IF I GO...*
IF I GO...
Let us analyze together two preponderant aspects in the question that is highlighted: the first of them refers to the word "case”, now portrayed by a noun, but which can also be applied with a conjunction value, whose notion is revealed by the aspect conditional.
When dealing with this same aspect, we naturally remember the conjunction “if”, basic, par excellence. Thus, let us analyze the following statements, whose discourses are manifested both by the use of the conjunction "if" and the conjunction "case":
If I go to the movies, I'll let you know in advance.
If I go to the movies, I'll let you know in advance.
Ratifying, both conjunctions reveal a hypothetical notion, a probable fact, that is, it can either happen or it doesn't. Thus, our intent is to make you understand and realize that the verb forms were applied in different ways, in view of the conjunction used.
In this sense, it should be stated that these are inherent times to the subjunctive mode, but different, applicable to particular cases, respectively demarcated by the future of the subjunctive (if I am) and present of the subjunctive (in case I go).
*Therefore, the construction “if I am” is not correct.
By Vânia Duarte
Graduated in Letters
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/o-vocabulo-caso-usado-como-uma-condicional.htm