I would wear (acceptable verb form) I had wear (NOT an acceptable verb form) => I had worn. "I should" and "I could" are NEVER written as contractions, only "I would". Hint: I'd = I had => must be followed by a past participle => I had broken, I had realized... I'd = I would => must be followed by bare infinitive => I would go, I would do => or by "have" + past participle => I would have gone, I would have done

It's a contraction of 'I would.' https://learnenglish.britishcouncil.org/en/english-grammar/verbs/modal-verbs/will-or-would

Great question, Benj! You're right, "I'd" in this case could only refer to "I would" or "I had," not "I should." Let's look at the sentence in the present tense: you wake up in the morning and stand in front of your closet. You think, "hmm, I think I'll wear this shirt today!" Of course, here, "I'll" = "I will." Now, if we look back at that moment from *now,* we have to change it to the past. 'think' becomes 'thought' and 'will' becomes... you guessed it: 'would!' I hope this is helpful for your understanding, and I hope you enjoyed the film! - Sarah https://www.italki.com/teacher/3458152

Benj, yes should is never that contraction.It's 'I thought I WOULD wear a shirt today' one thing to remember is that it can't be 'had' because then the sentence would be past perfect so the verb would change to 'worn'. Another interesting thing you said in your description was 'maybe I'd say' which has kind of the same idea as your question.