(be) unknown (mark) (celebrate) his 3 Put the verbs in brackets in the correct tense form. It is a bit ironic that the royal date of birth which 1) (commemorate) by the British to historians for so many years 2) monarchs to this day. Future King Henry IV was born on Maundy Thursday in 1367. It was such an important day in the Christian calendar that his birthday 3) on Maundy Thursday, not on the actual date of birth. The holiday on which Henry IV was born was a moveable one so each year he 4). birthday on a different date. When he 5) - (mark) his fifteenth birthday, (give) gifts to fifteen poor people starting the current Maundy tradition of giving gifts according to the monarch's age. Before him kings and other members of the royal family 7) – (make) Maundy Thursday donations to 13, 50 or 200 poor people. By 1388, his wife as well as his sons 8) up the tradition. Thus, when Henry V 9) (crown), Maundy Thursday 10) (already / become) a sort of 'official birthday' for the king. he 6) (take)

It is a bit ironic that the royal date of birth which has been unknown to historians for so many years is now celebrated by the British monarchy to this day. Future King Henry IV was born on Maundy Thursday in 1367. It was such an important day in the Christian calendar that his birthday is commemorated on Maundy Thursday, not on the actual date of birth.

The holiday on which Henry IV was born was a moveable one, so each year he celebrated his birthday on a different date. When he marked his fifteenth birthday, he gave gifts to fifteen poor people, starting the current Maundy tradition of giving gifts according to the monarch's age. Before him, kings and other members of the royal family made Maundy Thursday donations to 13, 50, or 200 poor people. By 1388, his wife as well as his sons took up the tradition. Thus, when Henry V was crowned, Maundy Thursday had already become a sort of 'official birthday' for the king.

To complete the sentence, we need to put the correct tense form of the verbs in brackets. Let's go through each verb and determine the appropriate tense:

1) commemorate - we use the past simple tense because the action of commemorating the royal date of birth has already happened in the past.
2) are celebrated - we use the present simple tense because this is a general truth that continues to happen in the present.
3) was - we use the past simple tense because the action of his birthday being on Maundy Thursday happened in the past.
4) celebrated - we use the past simple tense because the action of celebrating his birthday on a different date happened in the past.
5) marked - we use the past simple tense because the action of him marking his fifteenth birthday has already happened in the past.
6) took - we use the past simple tense because the action of him taking up the tradition has already happened in the past.
7) made - we use the past simple tense because the action of kings and other royal family members making Maundy Thursday donations happened in the past.
8) took - we use the past simple tense because the action of his wife and sons taking up the tradition has already happened in the past.
9) was crowned - we use the past simple tense because the action of Henry V being crowned happened in the past.
10) had already become - we use the past perfect tense because the action of Maundy Thursday becoming an 'official birthday' had already happened before Henry V was crowned.

Therefore, the correct completed sentence would be:

"It is a bit ironic that the royal date of birth which was commemorated by the British to historians for so many years is now celebrated by the British monarchy to this day. Future King Henry IV was born on Maundy Thursday in 1367. It was such an important day in the Christian calendar that his birthday was commemorated on Maundy Thursday, not on the actual date of birth. The holiday on which Henry IV was born was a moveable one so each year he celebrated his birthday on a different date. When he marked his fifteenth birthday, he gave gifts to fifteen poor people, starting the current Maundy tradition of giving gifts according to the monarch's age. Before him, kings and other members of the royal family made Maundy Thursday donations to 13, 50, or 200 poor people. By 1388, his wife as well as his sons took up the tradition. Thus, when Henry V was crowned, Maundy Thursday had already become a sort of 'official birthday' for the king."