The Collins English Dictionary defines phonotactics as the study of the possible arrangement of the sounds of a language in the words of that language. Phonotactics is the branch of phonology that deals with the restrictions in a language on the allowed combinations of phonemes (contrastive sound segments). At its simplest, it is concerned with the freedoms and restrictions that languages allow in terms of syllable structure and sound sequences within a syllable, or the constituents of a syllable. Phonotactic constraints vary from language to language.

For example, the clusters /kn/ and /ɡn/ are not permitted at the beginning of a word in Modern English, but they are in German and they were permitted in Old and Middle English, hence the spelling of words such as knightand and gnome. Spanish, on the other hand, does not allow s+consonant sequences at the beginning of a word, while these sequences were allowed in Latin (and are allowed in most modern Romance languages). For this reason, Spanish speakers will apply some kind of a “repair strategy” when confronted with such a sequence. Most frequently, they will “save” the cluster by attaching an e before it and pronouncing, for example, “espray”.

Syllable

A common source of mispronunciations are phonotactic constraints related to the syllable. Syllables are units of organisation for sequences of speech sounds. Syllabic writing dates to the third millennium BC, so several hundred years before the first letters. Syllables are considered by phonologists to have a hierarchical structure. They are made up of a syllable nucleus (generally a vowel) which can be preceded or followed by consonants. The preceding consonants are called syllable onset,and ones that follow form the coda. The behaviour of speech segments may vary according to their position in the syllable. Languages vary in their syllabic structure. 

Syllable
Source: Gringer, Wikipedia
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English syllable

English allows the following monosyllabic words (words consisting of one syllable); the colon shows that the vowel is long):

Syllable complexity in English

We can see that the maximum size of the onset in English is three consonants, as is the maximum size of the coda. It is relevant to examine the kinds of consonants that can occur in different positions; these examples show the privileged status that liquid consonants enjoy as the second element in onset clusters, and the privileges that are associated with /s/ as the “outermost” (furthest from the vowel) member of clusters, both in the onset and in the coda. Phonotactic knowledge is generally not conscious, it demands consistent awareness-raising. Many of the above combinations are not possible in Spanish and Spanish speakers will therefore have difficulties pronouncing them. 

Spanish syllable

The syllable structure in Spanish is simpler, that is to say, Spanish allows fewer segments in the syllabic constituents than English does. However, many phonological processes – especially the weakening of consonants in syllable coda – are best described in the light of syllable structure. 

A sequence of two consonants forms a syllable onset if the first consonant is a stop or /f/ and the second a liquid (in Latin these groups were known as muta cum liquida), e.g. o-tro (other), Á-fri-ca (Africa), and car-ta (letter), al-to (tall). 

Spanish is quite restrictive with regard to codas. Very few consonants are common in the coda. These are the dental and alveolar consonants; n, l, r, d, sand z, as in ciu-dad (city), car-ta (letter). Stops are much rarer and often simplified or weakened in colloquial speech, e.g. ac-to (act), sig-no (sign). In complex codas, the second consonant is always s, e.g. trans-crip-ción (transcription).

It is interesting to note that while words such as vaho [ba-o] (mist), búho [bu-o] (owl) – where two adjacent vowels belong to two different syllables – are fairly frequent in Spanish, they are very rare in English.