In Finnish, the distinction between voiced and unvoiced stops, i.e. b vs. p, g vs. k, and d vs. t, is not at all as clear as in English. A Finn very often pronounces the word bitti (bit) almost as pitti. Such pronunciation is often criticized and ridiculed when used in public speech, but on the other hand, clearly voiced b and g are rare except in formal speech. The common pronunciation seldom causes real confusion. Even though e.g. pitti also exists as colloquial word (covering a small part of the meaning of English “pit”), the context normally resolves any ambiguity between bitti and pitti.
In English, the difference between “b” and “p” is partly in the voicedness of “b”, partly in the aspiration of ”p” in many contexts. In Finnish, b is usually only mildly (if at all) voiced, and p lacks any aspiration, so the difference is usually small.
Similar considerations apply to g and k. Although the contrast between d and t is a different issue, the t sound lacks all aspiration in Finnish.
If you use normal English pronunciation for p, t, and k e.g. at the start of a word, the aspiration may make them sound almost like ph, th, and kh to a Finn. In the midst of a word, they may sound like pp, tt, and kk—at least when a word is pronounced in isolation.
Originally, the Finnish language lacked the [b], [d], and [g] sounds. In loanwords, they were replaced by [p], [t], and [k], respectively, e.g. pekoni (bacon), timantti (diamond), kirahvi (giraffe).
Later, it became common to preserve them in newer loanwords in educated speech. Although this is now the norm, it is followed to a varying degree, and the pronunciation is seldom completely voiced. Since the sounds [b] and [g] appear in new loanwords only, they are thus markedly foreign when pronounced as fully voiced.
The sound d was artificially introduced into Finnish in the 18th century, and it is now relatively well-established. In standard language, the distinction between d and t is essential: it often makes a difference in meaning in purely Finnish words. For example, matot is the plural of matto (mat), madot is the plural of mato (worm).
When Finnish was formed as a written language, the consonant gradation counterpart to t was a dental fricative similar to voiced “th” in English. It was written as dh or d. Later, this sound vanished from most forms of Finnish and was replaced by different sounds in different dialects. Due to the influence of the writing system of Swedish and other languages, people started to read the letter d and the combination dh as the [d] sound. This became part of the standard language, and the spelling was unified to use d.
However, dialects were not changed. Later, the [d] pronunciation has become more common even in informal speech and can be regarded as part of standard spoken Finnish, though it is common to use one’s dialectal pronunciation instead.
Outside its role as a consonant gradation counterpart to t, the letter d and the sound [d] are somewhat foreign in Finnish. It is often replaced by [t], though e.g. pronouncing demokratia as [temokratia] probably sounds somewhat uneducated or outdated to most Finns nowadays.
In pronunciation, the difference between [d] and [t] in Finnish is not just in voicedness. Another, usually more important difference is that [t] is dental: the tongue touches the upper teeth.
Since the b and g sounds appear mostly just in formal speech when trying to pronounce loanwords “right”, they can be regarded as prestige sounds. They might be used instead of p and k just to sound educated, saying (and writing) e.g. grooninen instead of krooninen (chronic). This could mean parody or a simple mistake (hypercorrectism), and it is a common phenomenon in slang (e.g., biisi comes from English “piece”). Similarly, d may appear instead of t (e.g., daideilija as a parodic form of taiteilija “artist”), even though d is not as foreign as b and g.