幸亏 / 幸好 / 好在: luckily, fortunately
【高37】【高45】【高46】【高110】语气副词、假设复句 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
幸亏 and 幸好 name the lucky circumstance that prevented a misfortune, and the misfortune is then spelled out after 要不然 / 不然 / 要不 / 否则 (“otherwise”). English packs it into “luckily…, or else…”; Chinese lays the two halves side by side: the lucky fact, then what would have happened without it. 幸亏你提醒了我，要不然我就忘了。
幸亏 stresses who or what you owe it to (幸亏有你帮忙: “thank goodness you helped”); 幸好 records plain good luck (幸好今天没下雨). In speech they are almost interchangeable. What follows 幸亏 or 幸好 is always what saved us, never the misfortune itself: that did not happen, and it comes after 不然.
好在 has no “otherwise”: it consoles, pointing to the one bright side of a bad situation (好在他病得不重: “the good thing is that he isn't seriously ill”). It opens the clause that brings the good news.
All three are everyday words, at home on the radio and in print. The “otherwise” half is the 否则 / 要不然 of level 4: do not replace it with 所以.
Examples
幸亏你提醒了我，要不然我就忘了。
Xìngkuī nǐ tíxǐng le wǒ, yàobùrán wǒ jiù wàng le.Luckily you reminded me, or I would have forgotten.
幸亏有你帮忙，不然我真不知道怎么办才好。
Xìngkuī yǒu nǐ bāngmáng, bùrán wǒ zhēn bù zhīdào zěnme bàn cái hǎo.Thank goodness you helped; otherwise I really wouldn't have known what to do.
幸好今天带了伞，否则大家都要淋雨了。
Xìnghǎo jīntiān dài le sǎn, fǒuzé dàjiā dōu yào lín yǔ le.Luckily we brought an umbrella today; otherwise we would all have got soaked.
好在你现在也在北京，你们可以互相照顾。
Hǎozài nǐ xiànzài yě zài Běijīng, nǐmen kěyǐ hùxiāng zhàogù.The good thing is that you are in Beijing now too, so you can look after each other.
好在他病得不重，应该很快就能上班了。
Hǎozài tā bìng de bù zhòng, yīnggāi hěn kuài jiù néng shàngbān le.Luckily he isn't seriously ill; he should be back at work very soon.
幸好我及时收了手，不然连老家的房子都得搭进去。
Xìnghǎo wǒ jíshí shōu le shǒu, bùrán lián lǎojiā de fángzi dōu děi dā jìnqù.Luckily I stopped in time; otherwise I would have had to sink even the family home into it.
Common mistakes
✗ 幸亏我迟到了，错过了飞机。 → ✓ 幸亏我没迟到，不然就错过飞机了。 (what follows 幸亏 is what saved you; the misfortune avoided comes after 不然).
✗ 幸亏你来了，所以我没事。 → ✓ 幸亏你来了，不然我就有事了。 (the next clause is the misfortune avoided, introduced by 不然 / 要不然, not a result with 所以).