@many examples of Natvam

**vRkSa- वृक्षॱ m + /nAm
→ vRkSA- वृक्षाॱ + /nAm
→ vRkSAnAm वृक्षानाम्
→ !**vRkSANAm वृक्षाणाम्

**vRkSa- वृक्षॱ m + /TA
→ vRkSA- वृक्षाॱ + ina इन by TAGasiGas...
→ vRkSena वृक्षेन by AdguNaH
→ !**vRkSeNa वृक्षेण

**brahman- ब्रह्मन् m + /am
→ !**brahmANam ब्रह्माणम् by /Natvam

rudh रुध् + /tip
→ ru रु + na न + dh ध् + ti ति
→ runaddhi रुनद्धि
→ **ruNaddhi रुणद्धि "obstructs"

**dviS- द्विष् + /zAnac
→ dviSAna- द्विषानॱ
→ **dviSANa- द्विषाणॱ "that hates"

ApUrya- आपूर्यॱ + /zAnac
→ ApUrya- आपूर्यॱ + /mum + Ana- आनॱ
→ ApUryam- आपूर्यम् + ANa- आणॱ by /Natvam
→ !**ApUryamANa- आपूर्यमाणॱ "that is being filled in"

There are tons of complicated grammar pages written about this /Natvam change, how can I get used to make it automatically without thinking.

You have to train your tongue to NOT move down after r र् R ऋ RR ॠ S ष्, and to ALWAYS move down after N ण् NN ण्ण् T ट् D ड् Th ठ् Dh ढ्. To do tat, start with saying aloud —

apUryamANam अपूर्यमाणम्

taking care to leave your tongue SOMEWHAT CURLED BACK after you say the r र्, then it stays a bit retracted during y य् a अ m म् A आ, then the N ण् brings it down forward.

A zloka श्लोक for practice —

pazyaitAm pANDuputrANAm AcArya mahatIJ camUm |
पश्यैतां पाण्डुपुत्राणामाचार्य महतीं चमूम्।
vyUDhAn drupadaputreNa tava ziSyeNa dhImatA ||
व्यूढां द्रुपदपुत्रेण तव शिष्येण धीमता ॥

Say that one very slowly, making sure to leave your tongue bit raised after putr पुत्र् AcAr आचार् dr द्र् putr पुत्र् ziS शिष्, and to bring you tongue down forward after pAND पाण्ड् putrAN पुत्राण् vyUDh व्यूढ् putreN पुत्रेण्.