Is this sentence grammaratically correct?

"Although I already wished you one at Seminary this morning, I still hope you have a Happy Birthday."

correct. The only mistake here is the word "grammaratically"!

Maybe a bit too much capitalization, but I'm not sure that qualifies as grammar.

Should "a Happy Birthday" not be capitalized?

Happy birthday should not be capitalized. I wouldn't capitalize seminary either.

Yes, the sentence is grammatically correct. It is a compound sentence that combines two independent clauses using the coordinating conjunction "although."

To determine if a sentence is grammatically correct, you can follow these steps:
1. Read the sentence carefully to make sure all the words are spelled correctly.
2. Identify the subject(s) and verb(s) in the sentence.
3. Check if the sentence follows the rules of grammar, such as subject-verb agreement, proper use of punctuation, and correct word order.

In the given sentence, "I" is the subject, "already wished" and "hope" are the verbs, and "you" is the object. The sentence is correctly punctuated with appropriate capitalization and a period at the end.