ذَهَبْتُ خَلْفَ زَيْدٍ — I went behind Zayd.
خَلْفَ زَيْدٍ identifies where the speaker went, so it is an adverb of place.
وَقَفْتُ تَحْتَ الشَّجَرَةِ — I stood beneath the tree.
تَحْتَ الشَّجَرَةِ identifies where the speaker stood, so it is an adverb of place.
These examples establish the contrast. The detailed place-word inventory is not needed to understand the time rule in this level.