A prepositional phrase starts with a preposition and functions as -

• Prepositional Phrase:
- যে Phrase Preposition দিয়ে শুরু হয়; কিন্তু Adjective বা Adverb এর কাজ করে, তাকে Prepositional Phrase বলে।
- At the minimum, a prepositional phrase will begin with a preposition and end with a noun, pronoun, gerund, or clause, the 'object' of the preposition.

Structures: 
- Preposition + Noun, Pronoun, Gerund, or Clause.
- Preposition + Modifier(s) + Noun, Pronoun, Gerund, or Clause.

Examples -
- At home:
At = preposition; home noun.
- In time:
In = preposition; time = noun.
- By singing:
By = preposition; singing = gerund. 

More Examples:
- The book on the bathroom floor is swollen from shower steam.
- Which book? The one on the bathroom floor!
- The sweet potatoes in the vegetable bin are green with mold.
- Which sweet potatoes? The ones forgotten in the vegetable bin!

Source: English Two, HSC Programme, Bangladesh Open University.