我要介绍一下我自己

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Both are grammatically correct; but it's more natural here writting 我想介绍一下我自己/介绍（一下）我自己/自我介绍

大学的时候我主修生物学了。

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大学的时候我主修生物学/我（上）大学的时候主修生物学。

 Misuse of 了：one of the most common errors by Chinese learners. Note that 了 indicating ' HAVING DONE something' or 'somet HAVING HAPPENED. 了 has nothing to do with 'time' or 'tense'. It has something to do with 'aspect'.  

In Chinese, you do not need to give a mark or affix to the VERBs or ADJECTIVEs when something happened or was done in the past time; but you can give some information about time with OTHER words, phrases or clauses when necessary.

Wether or not use 了 is a tricky problem.

✔ 下雨了！ ------- you are reporting news：something just happened or you've just found.

✘ 下雨！ or means other.

✔ 我吃过早饭了。---------- you are giving an answer to or as if to the question 你吃过早饭了吗？

✘ 我吃过早饭。

Cf: ✔ 我吃过榴莲。--------- another use of 过, indicationg some special EXPERIENCES.

✘ 我大学的时候主修生物学了。--------- you are narating a story or your history, not answering a question or reporting news, so 了 is needless.

毕业以后，我在都柏林大学工作了。我是生物学研究助理的，所以我对分子生物学和生物学有很好的了解。我（业余时间）最喜欢的事情是学习外语//我最大的（业余）爱好是学习外语。虽然我的母语是波兰语，我能说一口流利的英语，还能说一点法语。 现在我住在中国，学习汉语。空闲的时候我很喜欢拍照//摄影//(拍些照片)，看书，烘糕点。

我要介绍一下我自己

女士您好：）

我覺得妳的中文非常的好，所以修改的地方也很少。
另外在書面的寫法少一點“了”，會顯得比較正式。

YOUR CHINESE IS VERY GOOD :)
Could I ask you what books do you like recently? 

我要介绍一下我自己

整体上写得不错，大意能够描述的比较清楚，加油！

我要介绍一下我自己