1 .

小李：呵呵，不是去他的结婚，你应该说去他的婚礼。   Hehe, bùshìqù tā de jiéhūn, nǐ yīnggāi shuō qù tā de hūnlǐ.   Hah ,it's not his marriage. You should say wedding instead.  

2 .

Tom:谢谢。那我该说什么呢?   Xièxie. Nà wǒ gāi shuō shénme ne?   Thanks. Then what should I say?  

3 .

小李："新婚快乐"是比较时髦的说法，就是希望你的朋友结婚以后每天都开心。   "Xīnhūn kuàilè" shì bǐjiào shímāo de shuōfǎ, jiùshì xīwàng nǐ de péngyou jiéhūn yǐhòu měitiān dōu kāixīn.   "A joyful marriage" is a popular expression, which means that you wish your friend a happy marriage life.  
“百年好合”和“白头到老”都是老词了，不过如果外国人能说出来的话，你的中国朋友一定会更高兴的。   Búguò rúguǒ wàiguórén néng shuōchulai de huà, nǐ de zhōngguó péngyou yīdìng huì gèng gāoxìng de.   But if they were used by you,a foreigner,your Chinese friend would be much happier.  
这两个词都是希望你的朋友和他的爱人永远在一起。   Zhè liǎnggecí dōushì xīwàng nǐ de péngyou hé tā de àirén yǒngyuǎn zàiyīqǐ.   Both of them convey your best wishes that hope the new couple could lead a everlasting marriage life.  

4 .

Tom:谢谢你，我记住了。“新婚快乐”“百年好合”“白头到老”。   Xièxie nǐ, wǒ jìzhu le. "xīnhūn kuàilè " "bǎinián hǎohé" "báitóu dàolǎo".   Thank you. I keep them in mind.