As titled, both of these words always sound similar to me. Sometimes, the stress of these words will shift. For example, "seventeen" will say "SEVENteen". I am not sure whether it is correct or not. If so, and why?
TLDR: Seventeen is /sɛvənˈtin/ but seventy is /ˈsɛvənti/. (However, those are not phonetic transcriptions.)
Regarding your edit, you are right than the stress varies in seventeen. Normally its last syllable is stressed, but sometimes other factors can change this.
For example, when it forms part of a recited sequence of numbers like when counting up to twenty, stress normally shifts to the first syllable. That way the part that varies dominates.
Stress and Aspiration
The honest answer to “how many ways do these words get said” is a great many. However, both words have a cluster of related pronunciations that vary only slightly between region, speaker, register, and environment or context.
The common pronunciations in General American are:
Phonemic /sɛvənˈtin/ may be realized as phonetic [ˌsɛvɪ̈nˈtʰiːn], [ˌsɛvəⁿˈtʰiːn], [ˌsɛvn̩ˈtʰiːn] (amongst other things).
Phonemic /ˈsɛvənti/ may be realized as phonetic [ˈsɛvn̩di], [ˈsɛvəɾ̃i], [ˈsɛvɾ̃i], ['sɛvṇi] (amongst other things).
As you see, these two words differ from each other in multiple ways. What stands out most is that the /t/ phoneme in seventeen gets pronounced as an aspirated [tʰ] because it is stressed, while in seventy that same /t/ phoneme either reduces to an unaspirated and flapped [ɾ] in careful speech — or, more commonly, is lost altogether.
So stress and aspiration are your main distinguishing features here.
However, those aspects can sometimes be hard to catch in connected speech or over faulty connections. Indeed, over the telephone — especially when one or both parties are stuck using inaudibly supercompressed cell transmission — it is often difficult even for native speakers to distinguish between sixty and sixteen, or between seventy and seventeen.
For more details about the reduction of phonemic /ɛnt/ in this context, see Prof. Lawler’s answer to “How to pronounce twenty correctly?”, in which he concludes with:
Upshot: In American English, /'twəni/ is the normal pronunciation, /'twɛni/ is somewhat more formal and careful, and /'twɛnti/ is fastidiously careful.
The same applies here. Context can help with this if it’s there; for example, someone even quickly reciting the numbers by tens would always be understood because the ‑teen versions wouldn’t make sense there. Imagine kids rapidly counting off by tens while playing hide-and-go-seek; their sequence of:
- ten, twenty, thirty, forty, ﬁfty, sixty, seventy, eight, ninety, a hundred
becomes a somewhat blurred but completely unambiguous:
The more background you give in your question, the better we can answer it. I cannot tell from your profile what your country of origin or native language might be.
It may also be that you are not hearing the different phonetic allophones of the same underlying English phonemes as “falling into the same bucket”, so to speak.
But without more information from you, we cannot guess what more you need to know for this to become clearer for you.