In Bereshis 2:15 Adam is placed in Gan Eden to work and guard it. Guard it from what or whom?
The Ibn Ezra says - in his initial explanation - to guard it from animals so they don't enter and sully the garden:
ולשמרה מכל החיות שלא יכנסו שם ויטנפוהו
The Seforno says - if I understand correctly - to guard the fruit/trees? from rotting
ולשמרה. שלא תפסד בהתכת הליחות השרשי הנתך בחום הטבעי וזה כי אותם הפירות הנכבדים היו מולידים תמיד תמורת מה שנתך בלתי עפוש
The Ramban on verse 8 and 11 says - based on the Medrash that it refers to the sacrifices - since the word to guard is used in reference to the sacrifices.
לְעָבְדָהּ וּלְשָׁמְרָהּ אלו הקרבנות שנאמר (שמות ג יב) תַּעַבְדוּן אֶת הָאֱ-לֹהִים הדא הוא דכתיב (במדבר כח ב) תִּשְׁמְרוּ לְהַקְרִיב לִי בְּמוֹעֲדוֹ
But all the other commentators explain that since there was nobody to guard it from, it refers to keeping the Mitzvot; the commandments they were instructed.
לְמֶהֱוֵי פְּלַח בְּאוֹרַיְתָא וּלְמִנְטַר פִּקוּדָהָא
"To work in Torah study and guard the commandments"
Ibn Ezra - second explanation:
ויש אומרים כי כן פירושו לעבוד מצותו והמצוה לא תעבד
"Some say that it means to do the commandments and [ensure] the [prohibitions] don't get done".
The Ohr HaChaim summarizes:
לעבדה אלו מצות עשה ולשמרה אלו מצות לא תעשה
"To work: the [do] positive commandments, to guard: [to not do] the prohibitions".
(See all the above in Hebrew here.)
Other bible translation say: God took the man and settled him in the garden of E'den to cultivate it and to take care of it.
The conclusion is to fear God and keep His commandments. This is the whole of "man" or Adam. Adam was placed in the "garden" to work and keep His commandments. Therefore, the "garden" must be God's commandments.
Ecclesiastes 12:13 JPS (13) The end of the matter, all having been heard: fear God, and keep His commandments; for this is the whole man.
"Eden" means "delight, pleasure". Happy is the man ....whose delight (Eden) is in the law (garden) of the LORD. This is a spiritual place where man knows and walks with God in the "garden" of working and keeping His commandments.
Psalms 1:1-2 JPS (1) Happy is the man that hath not walked in the counsel of the wicked, nor stood in the way of sinners, nor sat in the seat of the scornful. (2) But his delight is in the law of the LORD; and in His law doth he meditate day and night.
Thank you for reading.
Vilna Gaon says to guard it from Nachash entering. He failed and the rest is history..