Hello i'm korean student. I learn Korean grammar in the school to have best grade. 잠궈 놔서 is wrong in grammar. 잠가 놔서 is correct. 잠그다 > 잠가 + ㅏ > 잠가 here ㅏin 가 is omitted. 놔서 = 놓아서's condensed version. It means state of something. someone has made something into a state. and vt + 아/어 놔서 usually means "because something is in the state of~" so '잠가 놔서' means "because someone has lock it" i think that 와이파이를 잠가 놔서 means because they lock(password) the wifi. I'm not so good at english writing but i wish it would be a little help for you.

To add a bit, 놓다, 놓아서, 놔서 are useful expressions widely in use much like 주다, 줘서 and 두다, 둬서. -(아/어) 주다 is for "(do smth) for someone", while -(아/어) 놓다 is for "(get smth) ready/available for someone/thing". -(아/어) 두다 is like "(do smth) for one's own use later", like -(아/어) 놓다 as a contingency provision for the future. So, if your son has been admitted to a college and needs a large sum of tuition, you can say: 1. 아이가 대학에 가서 등록금을 마련해 줘야 해. 2. 아이가 대학에 가서 등록금을 마련해 놔야 해. * 등록금 = tuition; 마련하다 = prepare, come up with. #1 sounds like you'll get the money and give it to him, whereas #2 means get it and make it ready for his use. Another interesting note is about 잠궈 놔서. 잠궈 is actually a wrong spelling - it should be 잠가 놔서. The derivation is 잠그다 + (아/어) => 잠가. Only if it were 잠구다 it would become 잠궈, but the root form is 잠그다. This is one of those odd cases that most people seem to get wrong in usage. Even Talktomeinkorean.com, a famous language teaching site, slips up with its usage here. Anyway, no matter how many people get it wrong, 잠가 놔서 is the right way (the language standards clearly say so).

I believe it is from 잠궈 놓아서 --> 잠궈 놔서. 놓다 usually means "to put, to leave". So people "leave the door locked" : 사람들이 문을 잠궈 놓다 --> since people leave the door locked : 사람들이 문을 잠궈 놓아서 ---> 잠궈 놔서. If you want to use it as past tense form, it'd be 사람들이 문을 잠궈 놨어 something like that. It has nothing to do with 나누다.