this one will be a little too much for a beginner because i believe here comes subtlities of Hindi Dekhna देखना- to see Dekh lena देख लेना- may also mean to see, but there is difference, also dekh lena may mean some other things too. they also are not interchangable many times except for the case where it means 'to see' for e.g. dekh lo, agar aana ho to. (think about it, if u want to come) देख लो, अगर आना हो तो. here dekh lo means think about it/or have a thought. dekh lena may also mean to examine/inspect sir, copies dekh lo take another case जो दिख नहीं रहा, उसे भी देख लेना दिव्य दृष्टि कहलाएगी। which is not visible, seeing(to be able to see) that too will be called divya drishti. here 'dekh lena' means 'dekh paana' i.e. to be able to see. last i can remember dekh lena is also used to indiacate holding grudge against someone e.g. tujhe to main dekh loonga- literally 'i will see you' :D i have missed some cases, someone might add them up, and i am totally dissatisfied with this answer because still sentences are popping in mind and i m unable to articulate them properly.

For your information ' dekhnaa ' is the root verb. It means To see / to look / to show / to exhibit (something) / to watch. However, usage of ' dekhnaa ' differs, depending on the situation and expression What you wish to convey. If you are writing / using ' dekh ' = See (this or that). More you use in ' Order ' Form. as well as ' Request ' or ' loving ' Form. ' dekh lenaa ' = As you wish to express something which is going to happen i.e. Future one use this sturcture. This is simple expression. However, you can also use this when someone says ' to teach a lesson ' or ' dekh loonga '. This tells that the ' WARNING' is given to someone. Ex: 1. Dekh lenaa ye (the above) spashtikaran aap kay samaz may aaega. 2. Eetna samazaa ne kay baad bhi agar aap ko ye samaz may nahi aataa hai to main tumhe ' dekh loonga ' Can you make out What the above 2 sentences mean ?