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Here's a sentence I made up:

"He seemed to had not understood what I had said to him"

Is this sentence correct? I tried searching for similar sentences by putting quotation marks around 'seemed to had not', and out popped roughly 5-6 results, but that doesn't seem to be that many, especially because some of those could've been mistakes, and I couldn't find any questions like this.

Also, assuming it is correct, if I change the position of 'not', like so:

"He seemed to not had understood what I had said to him"

Would it still be grammatical?

No matter where you put the "not", a statement with "to had" isn't grammatical. The construction you are using is "seem" + to-infinitive. The infinitive for the verb have/had is "to have", not "to had."

This is discussed in detail on the BBC "Learning English" website:

seem / appear to + infinitive

After seem and appear we often use a
to + infinitive construction ( or a perfect infinitive construction
for past events).

...

So what you should say is either of:

1. He seemed not to have understood what I had said to him.




2. He seemed to have not understood what I had said to him.




3. He seemed to not have understood what I had said to him.

The "not" could really go in any of those 3 places, but the first possibility sounds smoother and more idiomatic. The last sentence sounds the least natural to me, even slightly awkward.

No,

He seemed to had not understood what I had said to him.

is not grammatical at all, and neither is your other construction.

Here's what you should use:

He seemed not to have understood what I had said to him.

to has to be followed by a bare infinitive or perfect infinitive:

He seems to understand. [bare, present]

He seems to have understood. [perfect infinitive, past idea or tense]

The perfect infinitive is have + the past participle.