Questions about example sentences with, and the definition and usage of "Seventeenth"

A: With relevance to the 17th century. The 17th century was a time period of the past. When you say "related" that means that there is something in common from now (20th century aka 2016) that is the same with something from the 17th century (aka 1700's)

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A: × It is said that it was after seventeenth century, when the colonial rule by England was strengthened, that English began whelming into Ireland.
✓ It is said that it was after the seventeenth century, when the colonial rule by England was strengthened, that the English language began whelming into Ireland.

× Additionally, in 1840s the Potato Famine, which was said to have halved the population of Ireland, making mass starvation and exodus, determined the language replacement from Irish to English.
✓ Additionally, in the1840s the Potato Famine, which was said to have halved the population of Ireland, making mass starvation and exodus, determined the language replacement from Irish to English.

× Even now, after independent from the United Kingdom, most Irish people’s mother tongue is English, while Irish is the language learned only at school.
✓ Even now, after the independence of the United Kingdom, most Irish people’s mother tongue is English, while Irish is a secondary language learned at school.

A: He is the seventeenth disciple

A: Sounds fine :)

A: Century*

A: It sounds natural, but be careful with the punctuation points! You are creating a huge run-on sentence.