At the beginning of lockdown I felt very sad as I could not do most of the things I used to do like, playing football, swim or see my colleagues at the office as usual. Eventually I realized that not all are disadvantages, there are also cool changes like no longer waking up early preparing for going to work, I am now used to working from home, as I no longer stress about public transport or commuting to work. I also get to save money that I usually spend on lunch while at work.

For the first sentence you put, I placed a comma so I could properly link the dependent clause and the independent clause. A dependent clause is a sentence that relies on a complete sentence, an independent clause, to become a new complete sentence. For example, the sentence "When I went outside, it was raining." has 2 parts. The first one is a dependent clause because it cannot stand on its own; "When I went outside" is not a complete sentence. The second part, "it was raining", is an independant clause because the sentence stands on its own; it doesnt rely on anything else to become complete. When you add an independent clause and a dependent clause together, you get what we call a complex sentence. To sum this up, I corrected your first sentence because "In the beginning of the lockdown" is a dependant clause, and "I felt very sad because I..." is an independant clause, so I had to add a comma to make it a proper sentence. I'm not exactly sure what it is called, but for words like because, since, etc., you do not add a comma to link the sentences. When you said "I felt very sad, because I couldn't...", I removed the comma since it wasn't necessary. The image I shared has a list of most, if not all, of the words this applies to. When you said "I missed my colleagues and my office. But now I get used to working from home.", I removed the period entirely and combined the 2 sentences. For the words for, and, nor, but, or, yet, and so, they are able to connect 2 sentences by putting a comma before them and removing the period. For example, "I woke up early today, so I had to drink more coffee". Without the "so", it would be "I woke up early today. I had to drink more coffee." You can use the acronym FANBOYS (For And Nor But Or Yet So) to help you remember. I hope this helped you, and good luck.