Whether a word is acquired with ease or with difficulty depends on a number of factors such as pronounceability, orthography, length, morphology, grammar, and semantic features, which are thoroughly described by Schmitt and MacCarthy (1997:143-153) as follows:
PRONOUNCEABILITY

A number of studies demonstrate that the learners’ L1 system affects acquisition of a foreign language in various ways. It may be responsible for the learners’ inability to discriminate between sounds in a foreign language. Students may experience difficulties related to individual phonemes or phoneme clusters as well as in suprasegmental features of the language. In general, if the phonemic system of the target language is similar to the phonemic system of the students’ L1, the students will have less difficulty in learning the language.
ORTHOGRAPHY

The correspondence between pronunciation and spelling may also influence the learnability of a word. This is particularly true in case of English since the script of an English word may not leave the learner any clues as to how the word should be pronounced. For instance, the English letter “o” has a number of possible pronunciations such as in love, chose, woman, women, or odd. A different L1 writing system may also adversely affect the process of learning a foreign language, which may be a problem in case of native speaker of Chinese, Japanese, Russian, or some Semitic languages. Besides learning pronunciation and meaning of a new word, the speakers of these languages will also have to master an alphabet with which they may not be familiar.
LENGTH

It would seem that the longer the word, the more difficult it is to remember. However, Rodgers (1969) suggests that the length of a word is not a significant variable. According to another study carried out by Gerganov and Taseva (1982), Bulgarian learners of English memorize more easily one syllable words than longer words. In 1982 Coles discovered that the length of a word had a strong effect on its recognition. Coles’s findings suggest that shorter words are therefore easier to learn. This argument can be countered with morphological transparency, which is particularly evident with longer words. A long word can consist of many familiar morphemes, e.g. unavailability or unintelligible. There is no reason to believe that these words would be more difficult to learn than short one-syllable words, provided that the learner is familiarized with all of the morphological components. Word learnability is more affected by the frequency of exposure to the word. Although one may argue that the length factor is important in word acquisition, there is however no method of properly isolating this variable. For this reason it is hard to attribute the difficulty of learning a particular word to its length in a learning situation.
MORPHOLOGY

Multiplicity of forms of a word may cause a potential learning problem due to the complexity and increased load that needs to be memorized by the learner. This includes features like irregularity of plural, irregularity of tense, or gender of inanimate nouns. Knowing English affixes will certainly help students with composing and decomposing words. Derivational suffix such as pre- can be very easily understood by the learner in the word such as preview or prenatal. However, knowing affixes does not always guarantee proper comprehension of a word. Descriptive transparency is a special case of morphological difficulty in comprehension. The word outline does not mean out of line. Since the word consists of meaningful morphemes, the learner may assume that the meaning of the word equals the sum of meanings of its components. This is, however, not true in case of descriptive transparency words because the components are not real morphemes.
GRAMMAR
A number of research studies found that certain grammatical categories are more difficult than others. Nouns seem to be the easiest parts of speech, followed by verbs and adjectives, with adverbs being the most difficult to learn. Since English is not an inflectional language and English nouns do not take on any affixes except for plural or opposite forms in some cases, the foreign learners may find them the easiest to learn. Class conversion is a frequent phenomenon in languages such as English, which may constitute some problems in word comprehension and learnability, especially in the case of verbs.
SEMANTIC FEATURES OF THE WORD

The semantic properties that may interfere with learning process rendering it more difficult have been identified as:

  • Abstractness – it is generally assumed that abstract nouns are more difficult to learn than concrete nouns due to the more complex nature of the former. This may not, however, be the case. Learners of all ages do not display problems with understanding such abstract concepts as day, week, colors, or love. These concepts have been already developed in the learners’ L1 and they may require learning only a new form.
  • Specificity and register restriction. According to the study by Blum and Levenstom (1978), foreign learners tend to use general terms that can be used in a number of contexts, sometimes even overgeneralizing and ignoring restrictions concerning register and collocational restrains.
  • Idiomaticity. Idiomatic expressions are very specific to a language and their meaning cannot be easily induced from their components. Non-native speakers of English are more inclined to use the word decide instead of to make up one’s mind. This is due to the fact that the later is typical of English and not necessarily of the learners’ L1.
  • Multiple meanings. Polysemy as well as homonymy are rather prevalent in all languages and the difference between them is whether the meanings of a word represented by a single form are related or not. The study by Bensoussan and Laufer (cited in Schmitt and MacCarthy 1997:153) found that words with multiple meanings caused the largest number of comprehension errors. Learners may have a problem with discriminating between the different senses of the same word and may not bring themselves to use all of them.