Be aware of intonation and sentence stress.
Not all words in a sentence have equal stress, and generally only the "information" words (nouns and verbs) are stressed.
'Where's the 'pen I 'gave you?
'Where's the 'red 'pen I 'gave you?
Where's the 'red and 'blue 'pen I 'gave you 'yesterday?
The unstressed words (such as "the", "I", "you" and "and") don't carry as much "weight" as the stressed words. They become much smaller in length, and are almost abbreviated. For example, "and" becomes "un".
Changing stress
Sentence stress isn't "fixed" like word stress. In fact, you can stress words that are normally unstressed in order to highlight different meanings.
For example:
I 'love you. (Love, rather than just like.)
'I love you. (With the stress on I to highlight that it's me rather than another person who loves you.)
I love 'you. (And nobody else.)
Intonation
There are a couple of easy to remember rules about intonation. Usually our voices go up at the end of the sentence to show a question, and down at the end to show a statement.
Intonation is also important in "tag questions":
You know him, don't you? (With rising intonation on "don't you?" to show it's a question)
You know him, don't you. (With falling intonation on "don't you" to show it's a statement you expect the other person to agree with.)