Sometimes a noun in Gurbani ends with a Sihari, and there are several reasons for this.

Rule 1: ਸਬੰਧਕੀ ਪਦ (Preposition word)
One common reason a noun may have a sihari is when it functions as a ਸਬੰਧਕੀ ਪਦ (preposition).

For example, the words ਵਿਚ and ਅੰਦਰ appear in basic Punjabi without a sihari. In Gurbani, however, they often appear as ਵਿਚਿ and ਅੰਦਰਿ.

When a sihari is added to a word, it can indicate that the word is being used as a preposition.

Examples:

Normal form:
ਵਿਚੁ ਨ ਕੋਈ ਕਰਿ ਸਕੈ ਕਿਸ ਥੈ ਰੋਵਹਿ ਰੋਜ

• ਵਿਚੁ = ਵਿਚੋਲਾ ਪਨ (middle-man) (noun)

With sihari added:
ਨਵਾ ਖੰਡਾ ਵਿਚਿ ਜਾਣੀਐ ਨਾਲਿ ਚਲੈ ਸਭੁ ਕੋਇ ॥

• ਵਿਚਿ = ਅੰਦਰ (in) (preposition)

• ਨਵਾ ਖੰਡਾ ਵਿਚਿ = ਸਾਰੀ ਸ੍ਰਿਸ਼ਟੀ ਵਿਚ

Normal form:
ਅੰਦਰੁ ਲਗਾ ਰਾਮ ਨਾਮਿ ਅੰਮ੍ਰਿਤ ਨਦਰਿ ਨਿਹਾਲੁ

• ਅੰਦਰੁ = (hirda) (noun)

With Sihari added:
ਹੁਕਮੈ ਅੰਦਰਿ ਸਭੁ ਕੋ ਬਾਹਰਿ ਹੁਕਮ ਨ ਕੋਇ

• ਅੰਦਰਿ = (ਵਿਚ / in) (preposition)

Other examples:

ਜਲ ਥਲ ਮਹੀਅਲ vs ਜਲਿ ਥਲਿ ਮਹੀਅਲਿ

Without the siharees, these words simply mean water, land, and sky.
With the siharees, their meaning changes to in the water, in the land, and in the sky.

Without Sihari:

ਸਾਚੇ ਤੇ ਪਵਨਾ ਭਇਆ ਪਵਨੈ ਤੇ ਜਲੁ ਹੋਇ

• From the true Lord, the air was made, and from the air came water.

ਸਰ ਭਰਿ ਥਲ ਹਰੀਆਵਲੇ ਇਕ ਬੂੰਦ ਨ ਪਵਈ ਕੇਹ

• The pools are filled, and the lands have become green.

ਜਲ ਥਲ ਮਹੀਅਲ ਸਭਿ ਤ੍ਰਿਪਤਾਣੇ ਸਾਧੂ ਚਰਨ ਪਖਾਲੀ ਜੀੳ

• The beings of the water, earth, and sky are all satisfied.

In these examples, the words ਜਲ, ਥਲ, ਮਹੀਅਲ are simply nouns: water, land, and sky, and carry no additional meaning.

With Sihari:

ਨਾਨਕ ਤਿਸੁ ਬਲਿਹਾਰਣੈ ਜਲਿ ਥਲਿ ਮਹੀਅਲਿ ਸੋਇ

• Here, ਜਲਿ, ਥਲਿ, ਮਹੀਅਲਿ all have siharees. With the siharee added, their meaning changes to in the water (ਜਲਿ), in the land (ਥਲਿ), and in the sky (ਮਹੀਅਲਿ).

So, this line can be understood as:
“I am a sacrifice to that Lord, who resides in the water, in the land, and in the sky.”

Key takeaway:

• A noun without a siharee will always retain its basic meaning as a noun.
• When a siharee is added, one possible function is that the word becomes a ਸਬੰਧਕੀ ਪਦ (preposition).