This is an interesting question. Several comments and one of the answers here make it sound like any word can go through this radical kind of transformation if it is used in a different part of the sentence. This is not true!!
We can split words up, generally speaking, into two types for our purposes here. There are the kinds of words that you look up in the dictionary to find their meaning. These are often called content words, or lexical words. I prefer the second term. These words normally carry the essential meaning of a sentence. So if a two-year-old kid walks up to you and says:
then you know pretty much what they're on about. Notice that these types of words are easy to look up in a dictionary and easy to explain. They consist of common Nouns, Verbs, Adjectives and Adverbs.
The second group of words are what people call function words, or grammatical words. These words are difficult to explain, and you wouldn't be able to understand them by looking them up in a dictionary. They're usually small and they're the kinds of words that you would study in a grammar lesson, especially if you're learning English. Importantly, they rarely take stress. They consist largely of Pronouns, Auxiliary Verbs, Prepositions, Determiners, Coordinating Conjunctions and Subordinators as well as strange words like infinitival to. These words are actually not very important or very helpful at all in terms of communication. Note that if a two-year-old walks up to you and says:
.. you won't have the foggiest idea what they're trying to say! These words are the words that were missing, however, from the original sentence I want to go to the park.
Now the lexical words in a sentence usually contain the stressed syllables in that sentence. Stressed syllables in English give the impression of occurring at regular intervals. The effect of this is that stress and not the number of syllables is important for the perceived timing or duration of English sentences. The other effect is that the lexical words stand out more and the function words are less prominent.
In order to make this happen, English has a system for reducing the length of time it takes to say many of these function words that we've been talking about. This is so we can squash them in between the stresses. For many, if not most, of these words there are two forms (in terms of their sound). The usual form is the one used when:
- They aren't stressed
- They have another type of word or phrase following them, that they grammatically go together with.
These forms are called WEAK FORMS. They normally have a very short vowel. So the normal form for the Preposition to is the weak form /tə/ ['tuh' in the Original Poster's example]. However if to is used without a Noun directly following it it must be STRONG as in /tu:/. Note the vowel there has a colon-type mark. This indicates that the vowel is long. Also notice that the Noun is still there in the Original Poster's example, but it has moved to the beginning of the question and left to stranded at the end. We can draw up contrasts then such as:
- Who are you looking /fɔ:/?
- I'm looking /fə/ you!
Here we see strong and weak for respectively occurring without and with a following Noun. Note that strong for at the end of the sentence there is NOT stressed.
The rule is that prepositions like this are strong only if stressed OR if stranded. A strong form is definitely not an indicator of stress. We can have a full strong form vowel there without any stress at all.
As a rule of thumb then, if the word is the last word in the sentence it will be strong. If it isn't and it's not being stressed then it will be weak. [Pronouns
are an exception though, they don't take another word and so they're usually always weak when not stressed].
One last point to make is that not all function words have weak forms. For example the words in and on don't have weak forms ( - perhaps we should say that they have no exceptional strong form - they're already quite short ...). These words are always the same.
To try and answer the Original Poster's question, the words whose pronunciation is most likely to be radically affected by their position in a sentence, are function words. Not all function words are affected by this process however. The different forms that are used are called weak and strong forms. Weak forms are the standard pronunciations and the strong forms are the exceptions. We should note that when we call a word by name, for example if we want to talk about the word to, we use the strong form. This is because a) we're stressing the word, and b) because we have dislocated the word from any following items.
Hope this is helpful!