もしも is a 副詞（ふくし） (adverb) in Japanese that strengthens the base meaning of
もし ‘a bit’. Therefore, the literal meaning of
もしも is ‘even a bit’. Despite this, as
もし and
もしも are almost exclusively paired with sentences that use some sort of 仮定（かてい） ‘assumption’, or ‘hypothesis’ based expression,
もしも will often be translated as ‘supposing that’.
もしも will be used at the beginning of sentences that include almost any form of 仮定（かてい） expression. For example,
ば,
と,
なら, or ても.
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もしも、今年（ことし）中（じゅう）に引（ひ）っ越（こ）すのならば、僕（ぼく）が手伝（てつだ）ってやるよ。
Supposing that you move within this year, I will help you.
もしも仕事（しごと）で怪我（けが）をすると、現場（げんば）が止（と）まるので気（き）をつけてください。
Supposing that you get hurt at work, the site will be shut down, so please be careful.
もしも明日（あした）休（やす）みなら、一緒（いっしょ）に博物館（はくぶつかん）へいかない？
Assuming you have a day off tomorrow, do you want to go to the museum with me?
もしも彼（かれ）がいたとしても、今日（きょう）中（じゅう）には終（お）わらなかっただろう。
Supposing he even were here, don't you think we would not have been able to complete it by today?
Although
もしも is considered a stand-alone adverb, it is just a combination of
もし, and the 副助詞（ふくじょし） (adverbial particle) も.
もし and
もしも are most often used to express some type of result. This means that (B) will happen if (A) happens, or if (A) is true. However, たとえ is contrastive, and therefore asks ‘what the result’ of a specific situation may be.
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もし、また宿題（しゅくだい）を忘（わす）れたら親（おや）に電話（でんわ）をします。
Assuming you forget your homework again, I will call your parents. (Calling their parents will be the result of forgetting their homework)
たとえ、テストが難（むずか）しくても最後（さいご）まで諦（あきら）めないで頑張（がんば）ってください。
If, for example, the test is hard, don't give up, and do your best until the very end. (If the test is difficult, doing your best is not a result, just one of many choices that could be made)