Incorrect spelling, explanation: in order to make the ordinals, the rule says that it is required to add the appropriate indicator to the number in the figure. There are four indicators: st, nd, rd, and th. To the number 1 is added st to show that something is the first in the position. As a result, only 1st is errorless, not 1th.

Correct spelling, explanation: 1st represents the ordinal number the first that already existed in Old English as fyrst. In Middle English, there was a change and the word first was known as firme but this form did not survive. Since the 12th century, the word the first has existed in English and this form number 1 and the ending st represents the ordinal in the figure. Therefore, 1st is correct and 1th is incorrect.

Definition of 1st:
1st – the ordinal number indicating the first position of objects, just at the beginning,
The children sat in the first row.
I still remember the first time, I talked to Dido.
It was the first man, she fell in love with.

Phrases with 1st:
first aid – care given to an injured person before medical care can be obtained
They should receive training in first aid if they want to work there.
at first blush – at the first impression or when something is first considered
At first blush, his decision seemed ridiculous.

Incorrect spelling

Incorrect spelling