Learn גמרא דף יא: “אשכחיה רב ששת (seven lines before the Mishna) until the Mishna.
Clarify what the concept of חוקקין להשלים
See רש”י “חייבת which brings two גירסאות in the gemara. Rashi says this gemara is a proof that the side posts do not need to touch the top post.
What is the proof?
Is the proof only from the opinion of the Rabanan or also from Rebbe Meir?
What is the difference between the two גירסאות?
Why is an arched doorframe which starts to arch from the bottom of the side posts not obligated in mezuzah?
Is it only when lacking width of 4 tefachim with a minimum height of 3 tefachim or is it because a doorframe needs to have straight side posts and a flat lintel?
Learn “ח’ ר”ן “כיפה
Learn “ריטב”א “ר”מ מחייב
Learn רמב”ם ה’ מזוזה פרק ו הלכה ד
https://hebrewbooks.org/pdfpager.aspx?req=58957&st=&pgnum=244&hilite=
And שולחן ארוך יו”ד ס’ רפ”ז
https://hebrewbooks.org/pdfpager.aspx?req=9732&st=&pgnum=478