Grammar rule for the tilde:
If the word ends in a vowel, "s" or "n", the stress should be on the second-to-last syllable. If not, an accent belongs on the syllable that has the stress. If it ends in any consonant other than an "s" or "n", the stress goes on the last syllable. If not, the accent goes on that.
Sometimes, in words like "t�a" or "ciencias", the "ien" or "ia" are considered as one syllable. Also, there are words like "si" and "s�". The words mean different things if the accent mark is on a different letter, so be careful!
When you use the word "si" wihtout the tilde on the "i" you are using the condicional word "if". Example:
Si Mod Achilles postea en este hilo, yo vendr� y leer�.
Now, when you use the word "s�" with tilde on the "i" you are using the affirmative english word "yes". Example:
�Vendr� Mod Achilles hoy? S�, �l dijo que vendr� lo m�s pronto posible.
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thanks to iAstro129 and V_R4ng3r_V2 for it
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A similar case happen when you want to explain someting or ask for something.
When you use the word "porque" without tilde on the "e" you are explaining something and in this case "porque = because". Example:
Yo soy rico en runescape porque.....
When you use the word "por qu�" with tilde on the "e" you are asking for something. Notice that the word "por" is separated from "qu�" because you need to do it to give the sense of a question in spanish. Example:
�Por qu� Achilles es el mejor jmod de todos?
Of course, there are more grammatical rules for the tilde. it's a basic one!