Yes, he sensed Order 66 and Anakin's turn to the dark side. In fact, it's not hard for a Jedi Master like Yoda to do so. Ahsoka Tano even felt the disturbance in the Force when she actually heard Anakin's words right when he turned to the dark side. And mind you, this was when she was on route back to…
Master Yoda knew that Darth Vader was once Anakin Skywalker - the boy he sensed too much fear in to train way back in Star Wars: Episode I - The Phantom Menace. For many years it's been assumed Master Yoda was aware from the day Vader gained his cybernetic enhancements and signature wheeze.
Regardless of where anyone stands, there is one tiny thing that tends to throw a lot of people off, and that surrounds Palpatine. The emperor of evil is in close proximity to the Jedi, and not just any Jedi. Some incredibly powerful Jedi like Yoda, Mace Windu, and Obi-Wan Kenobi failed to realize what he was.
As for the evidence that came later, Yoda believed that the Jedi had already lost by participating in the Clone Wars. He wanted to prevent Order 66, but he also feared what would happen if the Jedi gave into fear over visions of the future. Still, it's unclear whether Yoda was right to ignore the warning.
Why Yoda Knew Sidious Would Try to Turn Anakin to the Dark Side - Star Wars Explained
Was Yoda aware of Anakin and Padmé?
They may not know the extent of Anakin's relationship with Padmé, or even that they are legally married, but it seems abundantly clear that Yoda, at the very least, was aware of Anakin's strong attachment to Padmé, and that those feelings were reciprocated, too.
Most viewers have forgotten that Palpatine seems to have been afraid of Yoda in Star Wars: Episode III - Revenge of the Sith - and even tried to flee from their duel.
Summary. Yoda recognized that he couldn't defeat Palpatine in one-on-one combat, as Palpatine's power in the Force was greater, and continuing to fight would likely result in his demise.
How powerful would Yoda be if he turned to the dark side?
Along with force lightning and force choke, there is also the brutal force rage. If Yoda had been able to use these powers in combat, he would've been extremely difficult to beat. With the connection that Yoda had to the force, the impact of him using dark side powers would mean unparalleled destruction.
Master Yoda is one of the most powerful Jedi to ever live, fighting alongside the other Jedi for hundreds of years and eventually coming to serve as a member of the High Council during the Order's last days.
There are certainly scenarios where Darth Vader would have the upper hand in a duel with Yoda, but in most cases, Yoda would be the more likely victor. Vader wields absurd levels of Force power during his dark side prime that would certainly rival Yoda's own abilities, but likely not exceed them.
R2-D2 was present when Padmé gave birth to Leia and Luke, so he knew Luke was Anakin's son – but what he didn't know was that Anakin became Darth Vader. In addition to that, Anakin's fall was surely a painful event in R2-D2's life, one that he definitely wouldn't like to remember or even talk about.
The canonical comic Star Wars: Darth Vader #7 confirms the younglings' deaths caused Anakin grief, self-loathing, hatred, and pain, fueling his descent even further into the Dark Side.
Why doesn't Yoda go and fight Vader? Because there were bigger threats. Yoda knew that Sidious was extremely powerful and that Obi-Wan would not be able to fight him. He sent Obi-Wan to Mustafar to defeat Vader instead, as Yoda hoped Vader could be defeated and killed.
Yoda had to shoulder that decision for years. However, he ultimately understood the importance of life, and the power he could have used to defeat Dooku would have been much better used to preserve two lives. Over the years, Dooku would commit countless atrocities and also suffer incredibly embarrassing defeats.
Even though Sidious mastered all forms of lightsaber fighting, Yoda can smash anybody (besides Luke in episode 7) in a lightsaber battle. Brute strength: Palpatine takes it. Palpatine can lash out with the force and do more damage than Yoda in one lightning bolt. Control: Yoda is the clear winner.
While this is not a fight that ever happened or ever will happen in a canon Star Wars story, it is one of the most interesting hypothetical matchups between two characters in the universe.
