となるとWhen, If, Once, As

となると is a strong expression in Japanese that focuses on highlighting that once some relatively high threshold is reached, (B) will occur. While となると may be seen in several different forms, the base structure is a combination of the particle と, the う-Verb なる 'to become', and the particle と repeated in its use of highlighting a certain outcome. Standard translations are 'once (A), (B)', and 'when (A), (B)'. A more literal translation can be thought of as 'once (A) happens, it is sure to follow that (B)'.

The other forms that となると may be seen in are ともなると, となれば, and ともなれば. The primary difference being that も adds slight emphasis. While ば and と do have slightly different nuances by default, in terms of this specific grammar point, they are used completely interchangeably.

となると may be seen following nouns, or verbs in any of their standard form.

も may also be used following the subject of ともなると sentences, and simply places emphasis on that subject.

Fun Fact

The emphasis that is seen in this grammar pattern is partly due to the function of the first と, and is one of the reasons that になると will not be used to convey a similar meaning. に and と are regularly used in this way in Japanese, with と almost always indicating that the outcome of something is unavoidable, cannot be controlled, is as to be expected. に merely focuses on the transition from (A) to (B).