تَحْتِ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – تَحْتِ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

التَحْتُ is the natural state of this definite noun and becomes التَحْتِ due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

التَحْتِ loses it’s definite article, becoming تَحْتِ due to it being a possessed noun

Beneath, Underneath

That which is below, The lower part, Beneath, Slope, Declivity of a mountain, Under

Surah/Below are the 48 verses in the holy Quran containing the genitive, possessed nounتَحْتِ
Verse
And give glad tidings to the believers and the righteous that for them will be gardens under which rivers flowوَبَشِّرِ الَّذِيْنَ ءَامَنُوْا وَعَمِلُوْا الصَّالِحَاتِ أَنَّ لَهُمْ جَنَّاتٍ تَجْرِي مِنْ تَحْتِهَا الأَنْهَارُ
Every time they will be provided with a fruit therefrom for provision, they will say “This is like the provision of before”كُلَّمَا رُزِقُوْا مِنْهَا مِنْ ثَمَرَةٍ رِزْقًا قَالُوْا هَذَا الَّذِي رُزِقْنَا مِنْ قَبْلُ
2-25and they will be given things in resemblance, and for them therein purified spouses and they will abide therein foreverوَأُتُوْا بِهِ مُتَشَابِهًا وَلَهُمْ فِيْهَا أَزْوَاجٌ مُّطَهَّرَةٌ وَهُمْ فِيهَا خَالِدُوْنَ
2-266
3-15
3-136
3-195
3-198
4-13
4-57
4-122
5-12
5-66
5-85
5-119
6-6
6-65
7-43
9-72
9-89
9-100
10-9
13-35
14-23
16-31
18-31
18-82
19-24
20-76
22-14
22-23
25-10
29-55
29-58
39-16
39-20
41-29
43-51
47-12
48-5
48-17
48-18
57-12
58-22
61-12
64-9
65-11
66-8
85-11
Their reward is with their Lord, eternal gardens, underneath which rivers flow, they will abideجَزَاؤُهُمْ عِنْدَ رَبِّهَمْ جَنَّاتُ عَدْنٍ تَجْرِي مِنْ تَحْتِهَا الأَنْهَارُ خَالِدِيْنَ فِيْهَا أَبَدًا
98-8therein forever, Allah SWT pleased with them and they with Him, that is for who feared his Lordرَضِيَ اللهُ عَنْهُمْ وَرَضُوْا عَنْهُ ذَلِكَ لِمَنْ خَشِيَ رَبَّهُ