一边……，一边……

一边...一边... is a frame with two hooks, so the second 一边 cannot be skipped. English "while" shows up once, which tricks learners into dropping the second tag. Without it, the second verb just hangs in the air and natives wait for the missing piece. Think of it like a pair of chopsticks — one stick alone picks up nothing. 我一边吃饭看电视 sounds cut off; 我一边吃饭，一边看电视 closes the frame.

了 stamps something as finished, but both halves of 一边...一边... are still moving. That is why 昨天我一边跑步，一边听歌 works — past time is fine, finished is not. The second you add 了, one side reads as a done deal and the sentence turns into a sequence. If one action truly ended first, split it: 我吃完饭再看电视. Same rule as 了 marking a completed action.

又...又... sticks two labels on one thing — what it is. 一边...一边... runs two actions side by side — what someone is doing. So 这家茶馆又安静又便宜 describes the place, while 他一边喝茶，一边看表演 describes the person. Swap 又...又... into the teahouse line and you turn a night out into a weird building that is made of tea and shows. Actions go with 一边...一边...; traits and states go with 又...又....

In casual talk, 一边...一边... often shrinks to 边...边..., especially with short everyday verbs. 边走边吃、边听边点头 — the 一 quietly vanishes and nobody hears a thing missing. The meaning does not change, so learn both forms instead of picking a favorite. Keep the full 一边...一边... for writing and formal speech; 边...边... is for chatting with friends and texting.