There are differences, they are not the same.
There are more differences in European Portuguese (pt-PT) than in Brazilian Portuguese (pt-BR).
"co" and "cou" sound different
In Portuguese, "ou" is a diphtong and it sounds very different from "o" (or "u").
Try this online converter of graphemes (words, syllables) into IPA: https://lsi.co.it.pt/g2p/
- Fico is /fˈiku/
- Ficou is /fikˈo/
Tonic syllable
Another important distinction is the tonic syllable.
The tonic syllable is very important in speaking proper and clear Portuguese, for reasons which are a bit long to explain.
fico and ficou have different stresses
Note the ` symbol in the IPA notation.
Fico has stress on the second-from-last syllable: /fˈiku/
Ficou has stress on the last syllable: /fˈiku/
The notion of oxytone, paroxytone and proparoxytone is very important in Portuguese.
This is something you will eventually have to learn and read about.
The tonic syllable gives the word "identity"
One of the major reasons is that we intuitively "identify" the word based on where the stress is. A bit similarly to how the "transposed letter game" still allows recognition of the intended word:
It deosn't mttaer in waht oredr the ltteers in a wrod are, (...) you can sitll raed it wouthit porbelm.
So, in pt-PT I find that the word "caderno" (has stress on the second-to-last syllable) is easier to understand as "aderno" or even "taderno" than as "caderno" or "caderno".
The word fico has tonic syllable on the first one, and ficou on the last one.
Following the sample from Schilive, they sound as different as the noun conflict and the verb to conflict.
Non-tonic syllables are weak
The tonic syllable has stress and must be spoken completely.
But non-tonic syllables can usually be reduced or even elided. When speaking, the vowels are often reduced; especially "e".
When speaking, you might hear "obrigado" as "brigado", "obrigade", or even "obrigad".
So, we might condense fico towards "fic'", ficou towards "f'cou".
Past tense has an acute accent
The 1st person plural (nós) in the past tense of verbs ending in -ar has an acute accent.
See the conjugations for ficar, cantar and rebolar.
- In the past tense, the 1st person plural has an acute accent: ficámos.
- In the present tense, the 1st person plural has no accent: ficamos.
Both have the stress in the second-to-last syllable.
The syllables are different, and they sound different to me.
In Brazilian Portuguese, they sound the same - an "open a", /a/.
In European Portuguese, they sound very different - an "open a" with accent, a "nasal a" without accent: perhaps /ɐ/?
- ca in ficámos sounds like the a in the English word Mumbai, and like the first a in capa.
- ca in ficamos sounds like the a in antes, or the first a in caminho (hear aimae and Sirasp, and then LinaCarm to hear it with /a/ in a clear Brazilian accent).