Where can I seek help when I cannot understand a research paper? Where can I seek help when I cannot understand a research paper? Instead of emailing the author since the paper is long time ago, I am preferring some discussion forum that people exchange ideas on the paper.

I am not a student in university so I cannot ask my professor but a software engineer in the industry.
There are a couple options within this site alone, and a couple other options that I’ve heard people having success with.

First of all, there have been several excellent answers on different stack exchanges about how to read papers in general, such as how to read a mathematics paper, how to take notes on research papers, how to remember what you read, and how to choose what to read.

Second of all, the stack exchange network is a great place to get answers to specific questions that you have about a paper or about reading papers.

If you have a general question about reading an academic paper that can be answered without reference to the paper, that will be on-topic on Academia SE.

If you have a specific question about the content of a paper about theoretical computer science, that can be on-topic here, at Math Overflow, and at Math SE depending on precisely what the question is. math.stackexchange is notably the only one on this list that accepts non-research level questions, so if you have a question that comes up in you’re reading that’s more foundational than research level, that’s a better place to ask it. There’s also CS SE  Which doesn’t have the “research-level” requirement but I’m not sure how strong their theory participation is.

The best sorts of questions for these SEs would be a question that can be answered in one go, rather than a question that starts a conversation. For example, questions along the lines of “I don’t understand the motivation/proof/concept in [passage],” would be on-topic. If you’re ever unsure if a question is appropriate, you can ask if you can ask that question on the corresponding meta site. To access the meta, click the meta button or add go to meta.[name].stackexchange.com

All of the aforementioned sites also have a chat that you can check out, and maybe find someone who is willing to sit down and talk about a paper with you for a bit.

Finally, if your question requires significant back-and-forth then a forum is potentially a better site. There are subreddits dedicated to mathematics and computer science, and there are a number of CS-oriented blogs or blogs with substantial CS readership that have open threads that invite people to raise questions that interest them. Slatestarcodex has such threads every other week, and I believe Scott Aaronson’s blog  has them occasionally too. I’m not an avid reader of the later, but it’s quite common for people tor raise papers that interest or confuse them for discussion and explanation on SSC. You used to be able to do this on lesswrong.com as well, though that community is largely defunct now from what I understand. I’m sure there are other blogs or communities that support such activities, and you can probably find them via simply asking around at those two blogs.