The proper placement of "only" and "just" is disputed. When modifying the subject, "only" and "just" are always placed before the subject: "Only I knew the answer." When used as adjectives they are always placed before the noun or pronoun they modify: "I lost my only hat." "He is a just man." When used as adverbs that modify all or part of the predicate they are usually considered to be "mid-position" adverbs and placed before the main verb, between an auxiliary verb and the main verb, or after a "be" verb that is being used as the main verb: "I am only interested in science."
However, many grammarians argue that when these adverbs modify a word in the predicate other than the verb, it is best to move "only" and "just" forward from their mid-position location to the actual word being modified to avoid possible ambiguous interpretations: "I only tossed the ball to you," "I tossed only the ball to you," "I tossed the ball only to you." As can be seen, the meaning does change, and if "only" is left before the verb there could be confusion if we are actually intending to modify one of the other words. So, moving the adverbs "only" and "just" forward seems logical. However, the problem is that this causes sentences to sound awkward, and often too formal sounding for normal conversational speech.
Avoiding formality and/or awkwardness in normal conversational speech is important and must be taken into consideration. Being that English is a "stress-timed" language, we speak more important words louder anyway. Thus, it is quite acceptable to make the meaning clear by just stressing the modified word: "I only TOSSED the ball to you.," "I only tossed the BALL to you.," or "I only tossed the ball to YOU." Therefore, in speech it is not really necessary to move the adverbs "only" and "just" to precede the word they actually modify.
Sometimes "only" and "just" are used to mean "recently" and are placed before the time adverb they modify: "He called me just/only yesterday." However, this also sounds rather formal for everyday speech and the meaning can be understood by stressing both "just" and "yesterday": "He JUST called me YESTERDAY."
In many cases there are no reasonable competing words in the predicate that could be modified anyway: In "I only have two dollars," "only" can only logically be referring to the "two," so why bother moving "only" forward creating an awkward sentence in the process?
Even in writing where one cannot stress the modified words common sense and context will most often clarify what is intended: "Dad, I only drove the car last night," which strictly speaking means that is all the person did with the car (only drove it) as opposed to some other activity like "wash it." However, common sense would likely suggest the meaning to be that last night was the only time the person drove the car, and context could further clarify the meaning: "Hey, have you been driving the car?" "Dad, I only drove the car last night." Saying (or writing): "I drove the car only last night" sounds a bit awkward (especially for a teen), and could be misinterpreted as: "recently drove."
There are situations where these adverbs modify the entire predicate: "I only wanted to make you happy," and habitual movement forward of these adverbs in an attempt to be more precise could cause the verb to mistakenly get left out: "I wanted only to make you happy."
In some instances, sentences do seem to compel movement forward of "only" or "just" due to the verb structure or the particular verb used: "I must live on only $10.00 per day," would sound very awkward if "only" were left before the verb, e.g., "I must only live on $10.00 per day." Saying, "He only died yesterday," might sound a bit absurd (How can someone have only died?).
With all of the above considered, it is apparent that it is usually best to keep "only" and "just" at mid-position to avoid formal sounding, unidiomatic speech in everyday language. An alternative approach is to just avoid the dilemma altogether by rewording the sentence using a relative clause: "Dad, last night was the only time that I drove the car."
Additional Note:
Michael Swan, Practical English Usage, Second Edition, p. 395:
"In speech the meaning is usually clear because the speaker stresses the part of the sentence that only refers to. Even in writing, the context generally stops sentences from really being ambiguous. However if really necessary only can be put directly before the object, complement, or adverbial expression that it refers to. This is rather formal. Compare ..."
