如果 synonyms for semi-formal written I can see from this post that in speaking 要是 is a good alternative.
However I am preparing for the 80 hanzi text required in HSK 5 and I feel like regardless of topic, I am over-using 如果.
My current candidates as synonyms to be used are:
假如 -- seems right
是否 -- seems to require a second clause after 是否你吃不吃.. then something else.
But are there multiple cases where these couldn't be used as a synonym for 如果?
There are way more synonyms in Chinese than listed. They are in most cases interchangeable depending on your desired degree of formality:






Explanation




如, 若
Originates from classical Chinese; formal. As in 如有垂詢 (should there be any enquiries)


假如, 倘如*
Can be either spoken or written.


假若, 倘若*
Fairly written.


假使, 倘使*
Fairly written.


如果, 若果
Can be either spoken or written.


要是, 若是
Contains the morpheme 'if' and the copula. Can mean 如果 or 如果是†.


～的話
Fairly spoken. Added immediately after the hypothetical statement.


假設
Meaning inclines to 'assuming that...' rather than 'if'.




* In general, 倘 is a written expression, more so than 假.
† If the original sentence already requires 是, e.g. 是我 (lit. is me), 是真的 (lit. is true), then 要是 means 如果是, as in 要是我 (if it were me), 要是真的 (if it were true). On the contrary, if the original sentence does not require 是, e.g. 你死了 (lit. you are dead), then 要是 means 如果, as in 要是你死了 (if you become dead).
Sometimes, for readability, it may be necessary to add coupling words 那, (那)就, or (那)便 before the result of the hypothetical statement:

如果你不服從上司，你便會被解僱。 lit. If you do not obey your boss, you will be fired.
假如時光可以倒流，那會多好！If only I could rewind time (, how great would that be)!


In English, 'if' can either mark a hypothetical statement, or a choice (as in 'whether'). Chinese however, makes a clear distinction between the two. That is to say, 是否 is not synonymous with 如果.


你看他是否睡著了？ See if (=whether) he is asleep.
如果他睡著了，別弄醒他。If he is asleep, don't wake him.