I would say: I'm not sure where the word 'yesterday' should be placed in the sentence.

I prefer "where" to "which" to speak about a location. Each sentence is grammatically correct. However, I would omit "where" and "which" entirely: "...the position of the word..." "...the position to place..." You do not doubt the "position" of the word. You know exactly where the word is. What you do doubt is the "correctness" of the position of the word. You could also say that you "have doubts about the position of the word" or you "have doubts regarding the position of the word". A better approach would be to use "placement" instead of "position". "Doubt" is not the best word here. "Question" would better describe your frame of mind than "doubt". It works well with both "position" and "placement". I suggest: "I question the placement/position of 'yesterday' in this sentence"

The best and most natural option is: ✔ "I doubt the position in which to place the word 'yesterday' in the sentence." (Sentence 3) This sentence correctly follows the structure: "the position in which to place...", which is a natural way to express uncertainty about placement. What about the other two? 1. "I doubt the position in which the word 'yesterday' to be placed in the sentence." ❌ (Incorrect) - "To be placed" is incorrect here. It should be "is placed" or "should be placed." 2. "I doubt the position where the word 'yesterday' is placed in the sentence." ✔ (Grammatically correct, but slightly unnatural) - "Where" can refer to position, but "the position where" sounds less precise than "the position in which." Tip: When talking about placement or position, "in which" is often better than "where" in formal writing. Also, use "to place" instead of "to be placed" when referring to where something should go.

Time goes at thr beginning and end of sentences and phrasds

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