Please help me fix the grammar in a birthday letter 祝你生日快乐!!! 对不起, 让你久等了
于一文: 你好!

If 于一文 is your friend, you should just use the first name 一文, and write: &quot;一文, 你好!&quot;
If you like to keep English favor in your letter, you can add 'dear' like in English letter and write: &quot;亲爱的一文, 你好!&quot;

祝你生日快乐!!! 对不起, 让你久等了。

If you need to apologize for lateness, better put it in first, and write  &quot;对不起, 让你久等了, 祝你生日快乐!!!&quot;

祝你年年有今日，岁岁有今朝。

This phrase is kind of dated, and only suitable for blessing a person in advanced age or at least in late middle age. If you must use it, put it in a separate paragraph to emphasize your blessing

祝万事如意。

No problem with this phrase

谢谢你的万事。

It is an attempt to translate &quot;thanks for everything&quot; but we don't use 万事 this way (the correct usage is another topic). The literal translation is &quot;一切都多得你&quot; ; However, &quot;thanks for everything&quot; imply &quot;thanks for everything you have done for me&quot; which shouldn't be omitted in Chinese, so the correct translation is &quot;谢谢你为我所做的一切&quot;

有机会的话，

It sounds like you and this person rarely have chance to meet. If it is not the case, you should add &quot;next time&quot; and write: &quot;下次有机会的话，&quot;

大家见个面。

It sounds too blunt, I suggest &quot;(希望)大家(能再)见面。&quot;

他不但是我的老师还是我的哥哥。

Do you mean: &quot;你不但是我的老师, 也是我的哥哥。&quot;?

我爱你♡

Chinese don't use to say &quot;I love you&quot; but you are not Chinese, so it is ok.
Here is my edition:

亲爱的一文, 你好!
对不起, 让你久等了, 祝你生日快乐, 万事如意!!!
谢谢你为我所做的一切! 下次有机会的话， 希望大家能再见面。
你不但是我的老师, 也是我的哥哥。
我爱你♡

Note 1:
In Chinese, we only use 万事 at the beginning of a phrase. It acts like an exclamation.
Example:
&quot;万事有我&quot; (I would deal with everything!)
&quot;万事都有例外&quot; (There's exception in everything!)
Note 2:
见个面 (meet once) --&gt; 能再见面 (can see each other again)
zyy wrote:

I would say 兄长 is better than 哥哥。

Consider replacing '哥哥' with '兄长' and write: &quot;你不但是我的老师, 也是我的兄长&quot; if you want to be more formal.
However, using '哥哥' is also acceptable, and it shows more passion-- consider the next phrase is &quot;我爱你&quot;