You probably learned the result complement 到 a long time ago, as part of words like 看到 ("to see") and 听到 ("to hear"). But other times it's less clear what the 到 really means. Like the difference between 买 and 买到 or 做 and 做到. What 到 adds is a clearer sense that something has really been achieved.

The difference 到 makes

Adding 到 after a verb makes it very clear that the desired result was achieved. If you looked and you saw, then 到 indicates that. On the other hand, not adding 到 makes it unclear whether or not the desired result was achieved. It can have the effect of making a verb mean "try to verb." So 做 without 到 is kind of like "try to do," but when you want to make it very clear that results were achieved or must be achieved, that's when you want to add 到.

Let's look at another example. The difference between 找 and 找到 is that 找 means "to search" (in other words, "to try to find"), whereas 找到 means "to find," that the search was successful, and results were achieved. So if one were to say "你一定要找," it would mean "you must try to find it" and "你一定要找到" means "you must find it."

Illustrative Examples

做 vs. 做到

我还没有做。Wǒ hái méiyǒu zuò. I haven‘t done it.
我下个星期可以做。Wǒ xià gè xīngqī kěyǐ zuò. I can do it next week.
我相信你一定能做到。Wǒ xiāngxìn nǐ yīdìng néng zuòdào. I believe you will be able to do it.

买 vs. 买到

我还没有买。Wǒ hái méiyǒu mǎi. I haven't bought it yet.
我没买到。Wǒ méi mǎidào. I didn't buy it.
这里很难买到进口食品。Zhèlǐ hěn nán mǎidào jìnkǒu shípǐn. It's hard to buy imported good here.

学 vs. 学到

我想学中文。Wǒ xiǎng xué Zhōngwén. I want to learn Chinese.
我们今天学了 “恭喜发财”。Wǒmen jīntiān xué le “gōngxǐ fācái”. Today we learned "kung hei fat choy."
在这个公司工作，我学到了很多东西。Zài zhège gōngsī gōngzuò, wǒ xuédào le hěn duō dōngxi. Working for this company, I learned a lot of things.

拿 vs. 拿到

你拿小包，我拿大包。Nǐ ná xiǎo bāo, wǒ ná dà bāo. You take the small bag, I'll take the big bag.
今天去拿婚纱，不知道能不能拿到。Jīntiān qù ná hūnshā, bù zhīdào néng bu néng nádào.Today I'm going to pick up the wedding dress, not sure if I'll be able to get it or not.
他偷偷进了她的房间，拿到了他想要的东西。Tā tōutōu jìn le tā de fángjiān, nádào le tā xiǎngyào de dōngxi. He secretly entered her room and got the thing he wanted to take.

想 vs. 想到

一定要想一个办法。Yīdìng yào xiǎng yī gè bànfǎ. You have to think of a method.
你想到了什么好办法？Nǐ xiǎngdào le shénme hǎo bànfǎ?Have you thought of any good methods yet?
我们都没想到他会这么说。Wǒmen dōu méi xiǎngdào tā huì zhème shuō.None of us thought that he would say it like that.

看 vs. 看到

我在看一些老照片。Wǒ zài kàn yīxiē lǎo zhàopiàn. I'm looking at some old photos.
看到这张照片，我想到了我的女儿。Kàndào zhè zhāng zhàopiàn, wǒ xiǎngdào le wǒ de nǚ'ér. After seeing this photo, I thought of my daughter.
我看到他了，在那里！Wǒ kàndào tā le, zài nàlǐ! I see him, over there!

追 vs. 追到

怎么追女孩子？Zěnme zhuī nǚ háizi? How do you chase girls?
那个女孩子，你追到了吗？Nàge nǚháizi, nǐ zhuīdào le ma? That girl, did you catch her yet?
如果你追到她了，我就请你喝酒。Rúguǒ nǐ zhuīdào tā le, wǒ jiù qǐng nǐ hējiǔ. If you can really catch her, I'll treat you to a drink.

了解 vs. 了解到

我不太了解这个人。Wǒ bù tài liǎojiě zhège rén. I don't really understand this person.
我们刚刚了解到一些新的信息。Wǒmen gānggāng liǎojiědào yīxiē xīn de xìnxī. We just understood the new information.
从他那里，我了解到了这个女人很多事。Cóng tā nàlǐ, wǒ liǎojiědào le zhège nǚrén hěn duō shì. From him, I understood a lot of things regarding this girl.

联系 vs. 联系到

我们怎么联系？Wǒmen zěnme liánxì? How do we get in contact?
我一直想跟你联系，但是一直没联系到你。Wǒ yīzhí xiǎng gēn nǐ liánxì, dànshì yīzhí méi liánxìdào nǐ. I always wanted get in contact with you, but I never could get a hold of you.
你知道怎么能联系到他吗？Nǐ zhīdào zěnme néng liánxìdào tā ma?Do you know how to get in contact with him?