Some grammar rules are handed down as absolutes, usually by people who were taught them the same way. A few of them describe real conventions of formal writing. Others were never rules of English at all, and following them faithfully makes your writing stiffer and harder to read than it needs to be. Four are worth unlearning.
The first is that a sentence must never begin with and or but. English writers have opened sentences that way for centuries, and careful ones still do, because a short connective at the front can carry a turn in the argument far more lightly than however or in addition. The real caution is about frequency: start every sentence that way and the writing begins to sound breathless.
The second is the ban on splitting an infinitive, which asks you never to place an adverb between to and its verb. It came from an attempt to measure English against Latin, where the infinitive is a single word and cannot be split at all. Put the adverb where the meaning is clearest: to really understand says something slightly different from really to understand, and clarity outranks the superstition.
The third is that a sentence must not end with a preposition. In practice, moving the preposition often produces a sentence nobody would say out loud, and the awkwardness costs more than the rule is worth. What did you ask for? is ordinary English; For what did you ask? belongs to a register almost nothing you write will need.
The fourth is that shall is obligatory with I and we. That distinction has largely faded from everyday use. Will now covers the future for every person, while shall survives mainly in offers and suggestions, as in Shall we begin?, and in legal or contractual writing. Using it elsewhere is not an error, but it will sound more formal than you probably intend.
What the four have in common is the gap between informal and incorrect. A choice too casual for a job application is not a mistake; it is register, and register is something you select on purpose depending on who is reading. Real errors are the ones that make a reader misunderstand you, and those deserve the attention these four myths usually absorb.