"Aamir saw Peter yesterday."
Translation:आमिर ने कल पीटर को देखा ।
When you are talking about people or something with its own agency, it is mandatory to use को (or use the dative form if it is a pronoun). Eg: उसने मुझे देखा(He saw me), मैंने भगवान को देखा (I saw God), जूलिया ने पीटर को देखा (Julia saw Peter) etc. This is also seen when you are asking questions. You say तुमने किसको देखा? (Who did you see?) instead of तुमने क्या देखा? (What did you see?)
For everything else, you can either use को or leave it out (in which case the noun becomes a direct object of the verb देखना). So, both मैने हाथी देखे and मैंने हाथियों को देखा are correct. [Notice that in the first sentence, देखना takes the gender and number of हाथी because it is its direct object. In the second sentence, देखना has no direct objects and so is seen in its default singular, masculine form]
The form with the को may be used to distinguish 'look' from 'see'. Also, the को form is often used with pets and other animals when you're not talking about them impersonally.
on a different topic. I just finished the lesson tree. I am going back and repeating all the lessons, and trying to use an online Hindi keyboard to write in Hindi. I am curious, does Duolingo Plus include a continuation of learning, or is it just a repeat of this tree without ads? I am eager to continue my learning.
Duolingo Plus does not include additional lessons. It is a way for you to remove ads on the app and gives you the ability to download lessons (to work on when you don't have an internet connection), unlimited hearts etc.
The tree will be updated soon and new lessons will be added but until then, redoing the lessons with a keyboard instead of the word bank is a good idea. Another thing a lot of people do is to attempt the reverse tree, ie, the 'Learn English from Hindi' tree.
On the app, press the Indian flag on the top left and click on '+course'. Click on 'more' and then scroll down to 'अंग्रेज़ी सीखें (हिंदी से)' (with an American flag).
On the web version, press the Indian flag and select 'Add a new course'. Change 'I speak English' to 'I speak Hindi' and then choose the English course.