我是做会计的 which is not past tense. 是 is verb. 做会计的 is noun which means person who doing sth. when 是……的 used in past tense, it means emphasized when/ how/ with who/ at where did u do sth. we already knew you did sth, and we want to know more specific for when/ how/ with who/ at where. so we can use 是……的。 我昨天是晚上八点回家的。 我是坐地铁回家的。 我是跟妈妈一起去的。 我的手机是在中国买的。

Generally speaking you’re not required to use 是...的. It mostly used for emphasis, and has nothing to do with tense. 我会保护你的 and 我会保护你 mean the same thing, but the first one is stronger in tone. 我是跟我爸一起去的 and 我跟我爸一起去了 mean essentially the same thing, but the first one is emphasizing the fact that I went with my dad. If you already knew I went on vacation, but didn’t know who I went with, I would use it to provide more information about a specific aspect of a known event. As for your specific example, double check with a native speaker, but both seem fine to me.

其实很简单，口语这两个表达是一样的，严格意义的汉语言语法可能会有一些区别，就和词与词的区别是一样的，通的部分很多，那么主要看你水平了，如果对汉语要求不高，不是专门做文学的，建议理解为两个是一样的！ 如果你是专攻这方面的呢，欢迎我们继续探讨~ 谢谢，希望对你有帮助~

There are two kinds of ' shi ......de'. Kind 1 ---- 你做什么工作/你是做什么工作的/你是做什么的/(你做什么的）？ ---- 我做会计工作/我是做会计的/我做会计的/我做会计/（我是做会计/我是做会计工作的） In 我是做会计的，'是’ is a verb, and '的' is an modal auxiliary, indicating an affirmative tone. Kind 2 ----- 明天你去工地上班！ ----- 我是不会[*]去的/我不会去的/我不去 [* 不会 has the stress；是has no stress] In 我是不会去的, 是 is an adverb/auxiliary, tensifier. 的，same as in 我是做会计的. If followed by a closely related clause, 的 is often left out. ---- 明天你去工地上班！ ---- *我是不会去(的），但是我弟弟可以去。 ---- 明天你去工地上班！ ---- 我[明天]是不会去（的）, 但是后天可以去。 Plus: 是 can be used as an adverb/auxiliary on its own, indicating a concessive mood. In this use, 是 always carris a stress, the strongest stress in the clause. ----- 小王学习很好。 ----- 他学习[是]好/他[是]学习好, 可是他不愿帮助别人（，所以他不能当选班长）。

Worth mentioning that nothing in Chinese ever has anything to do with tense. Even the closest equivalents, 了 and various verb+compliment structures （看到，吃完，etc) indicate completion or changes rather than time and can occur even in sentences that taka place in the future. As in, 我吃完饭之后，会打给你