I'm a native speaker of American English with Chicago/Ohio influences from my parents and have grown up mostly in the South. Here's how I do it:
I "kin" [can] go tomorrow, or, I "kən" [can] go tomorrow. These two vowel options are not necessary perceived as a binary choice, in other words, they easily blend into a hybrid vowel which is mostly subsumed by the nasal n sound. There is a moderately audible exhalation of air on the K, and once the jaws pulls up from the aɪ of the preceding word, the jaw stays in place. There is practically no vowel sound other than the humming of the n. It's really "kn".
Remember, in English we tend to really "hang out" on (extend) the m and n sounds, it's not like Spanish where the tongue or lips meet precisely, separate, and go on to another open vowel. Especially when the m and n sounds come at the end of words, they are treated practically like closed vowels in English.
Also the prosody of the affirmative phrase would tend to include subtle differences in pitch and emphasis. You'd hear some encouraging uptones, emphasizing "I" with a longer, upswinging tone, say, if one is volunteering oneself from among other people. Or if "tomorrow" is the operative assertion (as opposed to today, or next week) then "mər" of "morrow" is emphasized with a slightly higher (more friendly) pitch. These subtle changes in pitch and the lengthening of certain syllables call attention to the positive intent of the phrase.
Versus:
I "kæ(n)ʔ" [can't] go tomorrow. There is a more audible exhalation with the K sound. The n sound is more a nasalization of the æ, which vowel also requires a movement of the jaw. In terms of prosody, the "I" is quick, and there is a slight rhythmic emphasis on the syllables "can't", "go" and possibly the "ma:" of "morrow". To keep the pitch quick and low on "morrow", notice the vowel may open out from ər of the previous sentence to a:.
In addition to pronunciation differences, there are usually important clues in the surrounding choice of words, and facial expressions, telling you whether this is an affirmation or a negation.
I can't go tomorrow. [brow furrowed, squinty eye contact with listener]
I can't go tomorrow, but maybe [suggests an alternative].
I can't. [point blank refusal. All the earlier pronunciation qualities apply]
They can't go tomorrow because [explanation given with refusal].
I'm sorry, they can't go. [apology gives it away]
Who told you that? I can't go tomorrow, I have to be at work by 8. [heated refusal with reasons]
Or, [all the following are said with a relaxed, open facial expression-- lifted or neutral eyebrows, and a relaxed jaw and lips]
I can. [upbeat prosody]
I can hardly wait! [expression of enthusiasm]
I can go tomorrow if you want. [makes it a conditional offer]
They can go after they've finished eating. [another conditional affirmation]
These are all personal conversations. In other contexts such as the scientific commentary you reference, there should be other clues:
Fat cells can't reproduce themselves.
Fat cells can reproduce themselves.
With the second, positive affirmation, you are either going to hear the humming of the n sound of "can" for longer, with a: for the vowel, if the idea is being emphasized. Or the word will collapse down as I explained earlier, into "kn".
With the negative assertion you will hear more air with the K of "can't". You'll hear the nasally æ(n) sound with a glottal stop at the end.
There will probably be clues in the surrounding comments, as well.
Hope that helps.
First, it's "shwa". It's a Hebrew word, not a German one, so there's no reason for SCH.
Second, it's both a phone [ə] in IPA, and a phoneme /ə/ in English.
As a phone, it's got the sound of the final vowel in German Danke, of the first vowel in French Le Mans, or the first vowel in English the man. There is no shwa in Spanish or Italian.
Third, as a phoneme in English, /ə/ doesn't contrast with any other central vowel, so it has a lot of allophones: /ə/ [ɨ] [ə] [ʌ] (in increasing order of stress and decreasing order of speed), plus syllabic resonants [ṃ] [ṇ] [ḷ] [ṛ], before those consonants.
The best way I can suggest to practice the sound [ə] is to open your mouth to say an [e] (whatever that you think that is in your language), and then — while saying it, and without changing how your mouth or lips are positioned — move your tongue backwards toward the center of your mouth.
What you wind up saying is likely to be something close to a shwa.
Best Answer
Here is a typical English vowel chart:
As you can see, /æ/ and /ɛ/ are quite close to each other, and so have quite a similar sound. The difference is in the degree of openness: /æ/ is “near-open” and /ɛ/ is “open-mid”. If you want to make a clear distinction between the two, you need to practice, practice, practice. When I was learning phonetics, I found the best practice was to try to make long continuous vowels that go along the axes, like /iiiiiiiieeeeeeeeɛɛɛɛɛɛɛɛææææææææaaaaaaaa/. Once I was able to master making the entire continuum, it became easier to find individual points along the continuum.