Correct spelling, explanation: the number indicators th and st are added to numbers in figures to form the ordinals. Creating the ordinal number 1, we use the ending st as the ending in the word first. However, the ending th is used to numbers 4, 5, 6, and so forth. In Old English, the word was first known as fyrst, which also shows that nothing is changed. The letters st are indicators of ordinal number 1. As a result, 1st is correct, and 1th not.

Definition of 1st:
1st – the first is at the beginning of the list, before anyone or anything else,
Christ was born in the 1st century.
She won the competition, which placed her in the 1st position on the list.
King Pinto was the 1st ruler in this small country.

Phrases with first:
at first glance – when something is considered at the beginning,
At first glance, the plan seems to be successful.
at the first sign – any event that indicates that something has begun to happen,
They called the police at the first sign of trouble.

Incorrect spelling, explanation: the ending th is added to numbers, beginning from the number 4 in order to make the ordinal number. But st is an indicator for the number 1. English learners may make a mistake because there is only one number to which st is added. This is number 1. It is very important to remember that number 1 with st indicates the first, the word coming from Old English as fyrst. Therefore, 1th is incorrect and it should be written as 1st.

Incorrect spelling

Incorrect spelling