The sauce is actually the only part that matters
Most people treat Pad Thai like it is a noodle problem. It is not. The noodles just absorb whatever liquid hits them, so if your sauce is wrong the entire dish is wrong and there is no fixing it at the pan stage. I have ruined about six batches of noodles over the years by assuming I could fix the seasoning after the fact. You can't. Once the rice noodles take up the liquid, they are done. Start with 3 tablespoons of tamarind paste, not the concentrate from the tube. The paste has body and a darker, fruitier sourness. Mix in 2 tablespoons of palm sugar, 2 tablespoons of fish sauce, and 1 tablespoon of water. Whisk until the sugar is mostly dissolved. That is your base ratio. Taste it before you touch a wok. It should hit sweet first, then sour, then salt. If it is just sour you added too much tamarind. If it is flat you need fish sauce, not more sugar. I once bought a cheap bottle of fish sauce from a wholesale store and used it without tasting it first. The batch was hyper-salty and had a metallic undertone. My entire wok load came out tasting like soy sauce and regret. I had to add another full batch of sauce just to dilute it, which threw off the tamarind balance and made the noodles too wet. Now I taste every component before it goes into the mix. It adds forty seconds and saves a whole stir-fry.
Here is the thing nobody tells you: the sauce gets thinner the moment it hits a hot wok. The residual heat in the pan and the starch from the noodles will cut the viscosity fast. If you want a coating that clings instead of puddling at the bottom of the bowl, make the sauce slightly thicker than you think it should be when it is still cold in the bowl. A quick boil for about thirty seconds in the pan gives you enough reduction to get that glossy finish most restaurants aim for. Also, do not skip the acid check. Palm sugar varies wildly between brands. Some are almost neutral, some carry a heavy molasses bite. If your palm sugar is on the aggressive side you may need an extra half teaspoon of lime juice at the end to lift the flavor. I learned that after a batch from a particular supplier sat too heavy on my palate and I spent twenty minutes trying to fix it mid-cook. It was easier than I thought, but it should not have been necessary. One more note on ingredients: if you cannot find good tamarind paste your next best option is tamarind concentrate diluted with warm water at a one-to-two ratio by volume, then strained. It will not have the same depth but it works in a pinch. Cheap vinegar or lime juice alone will never replicate tamarind. It will taste like Pad Thai only in name.
The sauce keeps refrigerated for about five days in a sealed jar. The flavor actually settles and melds a bit after day two, which is worth noting if you are prepping ahead. Just bring it to room temperature and give it a good stir before using. Cold sauce hits the wok unevenly and can cause the noodles to seize up instead of coating properly.
Get the Full Details
