## 4 Discussion
### High Trajectories of T1 and T2  
The overall tonal register of L1-dominant Polish speakers learning Mandarin (L2) is significantly higher than that of TM natives because both T1 and T2 end at a high pitch point, 5. Taking T1 as an example, we observed a relatively higher tonal register of T1 produced by L1-dominant Polish speakers learning Mandarin (L2), whether in the first or second syllable, except for the tonal combinations of T1T3, T2T1, T3T1, and T4T1. This suggests that when a low-tone point of the tones precedes T1, it can normalize the high pitch of T1. The habitual higher pitch ranges of Polish may lead to a significantly high register of T1, even after being transformed into T-values individually. The issue with T2 lies in its higher register rather than its contour. We observed a deviation towards a high register for T2 in all tonal combinations, except for T2T2 and T2T3. Once again, T3 is suspected to contribute to this. It is worth noting that if T3 cannot be lowered, for example, in T3T2, T2 cannot be normalized if T3 itself is not produced well enough. Thus, the high trajectories of T1 and T2 can be mitigated when they co-occur with T3, a low tone. Ref. [49] conducted an experiment demonstrating that language transfer effects impact the F0 performance of learners acquiring Mandarin Chinese as a target language. For instance, in English, which is also a stress-timed language, F0 elevation typically occurs towards the end of interrogative sentences, whereas in Mandarin Chinese, F0 elevation spans across the entire sentence. These differences may lead English-speaking Mandarin Chinese learners to transfer their English-speaking habits to Mandarin Chinese, resulting in errors in intonation F0. Specifically, the contour tones T2 and T4 did not present learning difficulties in this study; instead, the challenge lay in tonal register. This is attributed to the rising contour being LH and the falling contour being HL in intonation. Despite extensive practice by L1-dominant Polish speakers learning Mandarin (L2) in sentence-level exercises, the mastery of tonal register remained elusive. Therefore, for L1-dominant Polish speakers learning Mandarin (L2), the application of F0 in Polish sentence intonation assists in acquiring tonal contour; however, due to Polish's habitual use of a higher F0 range, errors in pitch height persist whenever high-tone contours are involved in the target language. Furthermore, our research indicates that L1-dominant Polish speakers learning Mandarin (L2) exhibit a high register in the production of T1 and T2 in disyllabic words. This partially aligns with previous studies on Japanese, Korean, and German learners, which identified T2 as particularly challenging ([15; 16], for Japanese learners; [17], for Korean learners; [19], for German learners). Most challenges in these studies, as well as in ours, occur in the first syllable of Mandarin disyllabic words. More specifically, we found that T2 is often confused with T3 in the first syllable. Of course, more research is needed, as [PERSON] et al. (2013) [42], along with [PERSON] et al. (2019) and [PERSON] et al. (2024) [43; 44], found T3 to be the most difficult tone in both perception and continuous speech contexts. Interestingly, ref. [39] reported that L1-dominant Polish speakers learning Mandarin (L2) face difficulties with all four tones in the first syllable of disyllabic words. Our findings differ, possibly due to differences in the proficiency levels of learners. Ref. [39] studied L1-dominant Polish speakers learning Mandarin (L2) who had attended a seven-week Mandarin study program in Taiwan with over 60 h of instruction, whereas our participants have been studying Mandarin in a Polish sinology department for more than six months. In this study, comparing the tonal ranges of disyllabic words (2-3 T-values) among native Mandarin speakers, they appear relatively smaller compared to monosyllabic words (4.25-2.25 T-values), reported by [PERSON] et al.