Gurbani is filled with proves for the existence of Vyakaran, over and over. Anyway who denies it is either grossly misinformed, or just ignorant. In terms of particular shabads, there are a few that particularly stick out.
The following shabad is a clear example of the use of sihari on Nouns
ਸਚੀ ਬੈਸਕ ਤਿਨ੍ਾ
ਸੰਗਿ ਜਿਨ
ਸੰਗਿ ਜਪੀਐ ਨਾਉ
ਤਿਨ੍
ਸੰਗਿ ਸੰਗੁ ਨ ਕੀਚਈ ਨਾਨਕ ਜਿਨਾ ਆਪਣਾ ਸੁਆੳ
As you can see, sang, is used in two forms, both with a sihari and without a sihari. Here are the meanings of the two different words.
ਸੰਗੁ = ਸਾਥ (noun)
ਸੰਗਿ = ਨਾਲ (preposition)
Whenever the word is a ਸਬੰਧਕ then there will be a sihari added to the word. In this shabad,
ਸਚੀ ਬੈਸਕ ਤਿਨ੍ਾ ਸੰਗਿ ਜਿਨ ਸੰਗਿ ਜਪੀਐ ਨਾਉ, these two ਸੰਗਿ’s here mean nal ,.
Meaning, the true Sangat(ਬੈਸਕ ) is with those (ਸੰਗਿ=ਨਾਲ), who with(ਸੰਗਿ-ਨਾਲ) we will Jaap Naam
In the second tukh ਤਿਨ੍ ਸੰਗਿ ਸੰਗੁ ਨ ਕੀਚਈ ਨਾਨਕ ਜਿਨਾ ਆਪਣਾ ਸੁਆੳ, the first sang has a sihari, and mean’s nal. The second sang has an aunkaR and here it is a singular noun, meaning “sath.
Meaning that we should not keep friendship (ਸੰਗੁ-ਸਾਥ) with those (ਸੰਗਿ-ਨਾਲ) , who only care about themselves.
Clearly, this how Guru Sahib helped us Morakhs distinguish the meanings of the words, by adding Laga matra's.