- Through can be a preposition, an adjective, and an adverb.
- Through is the only formally accepted spelling of the word.
- Thru is an alternate spelling that should be used only in informal writing or when referring to drive-throughs.
As if all the confusion over the words through and threw wasn’t enough, modern English has piled on yet another homophone: thru.
Through vs. Thru
Through can be used as a preposition, an adverb, and an adjective. It has several meanings, including “from one side to the other,” “from beginning to end,” and “during an entire period”:
Through is the only acceptable way to spell the word in a formal situation. In informal situations, and especially when texting or instant messaging, the form thru is becoming increasingly popular:
There is one occasion when thru might be an acceptable choice even in formal writing. The word drive-through is commonly spelled with the shortened form of through:
Through vs. Thru: Examples
from Grammarly Blog https://www.grammarly.com/blog/through-thru/