mai (as opposed to mae which is a present form of the verb bod), is used with verbs to place positive emphasis and corresponds to 'that' in English. It can also be replaced in Welsh with taw, e.g.

One syllable words ending with b, d, g, f, dd, ff, th, ch and s do not need an accent on the vowel in the middle, e.g.

haf, nod, tref, cath, coch.

un causes a soft mutation if it is followed by a feminine singular noun, e.g.

un ferch

un gadair.

The mutation rules can be found by clicking here.

In Welsh, the preposition must have a suffix (an ending) to match the pronoun that follows it, e.g.

i + nhw = iddyn (preposition) nhw (pronoun).

There is no suffix if the preposition is followed by a noun, e.g. chwilio am y dillad, mynd i'r brifysgol.

A list of verb-nouns and prepositions can be found by clicking here.

Darn is a masculine singular noun, therefore dau must be used.  Also, remember that there is a soft mutation of nouns after dau and dwy, e.g.
dau ddyn
dwy ddynes.

In Welsh, many verbs are followed by specific prepositions. A comprehensive list of verb-nouns followed by their correct prepositions may be found by clicking here.

In this instance, anfon can be followed by at or i. We use anfon at if we are sending to a person, e.g.

Mae Rhys yn anfon neges e-bost at ei diwtor.

Anfon i is used if we are sending to a place, e.g.

Mae'n bwysig eich bod yn anfon eich cais i'r coleg yn brydlon.

The pronoun fy causes a nasal mutation in nouns beginning with p, t, c, b, d, g, e.g.

fy nheulu, fy ngwaith, fy mrawd.

A full list of mutations can be found by clicking here.